Voltage Gain Switch (Design Help)

[[email protected]]

Hi, recently I`ve started to work (ad honorem, as a Technician) on a
Mechanical Engeneering Lab that performs destructive tests. They have
tasked me (I`m in the Electronic Instrumentation Department) to design
an electronic circuit that will replace an obsolete Voltage Gain Switch
circuit. The obsolete circuit uses relays to change the operational
amplifiers with different gains. Basically we are 2 guys, I`m an
Electronic Technician on the first year of electronic Engeneering and
who's in charge is an about to graduate Electronic Engeneer.

The requirements are:

+-10V output voltage swing
Low Frequency, from 0 to 200hz
Low Cost
Easy-to-find ICs (ICs with high Availability)
4 selectable gains controlled by two bits
Null-Offset
High Precision Gain

The Technician before me did this circuit with 4 Operational Amplifiers
with different gains and an analog switch CD4051 or CD4066. This
circuit did not meet the requirement of +-10V output voltage swing, so
I have to redesign it. The gains are predetermined, I don`t have the
actual values right here, but the maximum was 5000. My idea is to
change the analog switch with one that does allow at least +-10V output
voltage swing, and my "boss" approved. The thing is that I tried to
find a replacement but couldn`t find one that`s available here in
Argentina.

To sum up what I`m asking for is a suitable IC, or a new idea on how to
make the circuit. Of course ideas to improve the original design are
always welcome. I know about amplifier design, but on the Technician
level, that is, I don`t know any practical or modern way of doing it,
just what the theory says. As you can see the requirements are
standard, not very demanding (I guess), if there's anything you think I
should consider please tell me about it.

Thanks in advance.

 

[Phil Hobbs]

Hi, recently I`ve started to work (ad honorem, as a Technician) on a
Mechanical Engeneering Lab that performs destructive tests. They have
tasked me (I`m in the Electronic Instrumentation Department) to design
an electronic circuit that will replace an obsolete Voltage Gain Switch
circuit. The obsolete circuit uses relays to change the operational
amplifiers with different gains.

(snip)

What's wrong with the relays? For that job, relays aren't a dumb idea
at all. The analogue switches will have some nonlinearity and some
temperature coefficient of resistance, whereas relays are nearly
perfect--their only drawbacks are cost, which you don't care about for a
one-off, and slow switching speed, which you also don't care about.

Cheers,

Phil Hobbs

 

[John Woodgate]

The gains are predetermined, I don`t have the actual values right here,
but the maximum was 5000.

That data is CRUCIAL for us to recommend a solution. 5000 is too much to
get reliably from one op-amp, even at 200 Hz.

My idea is to change the analog switch with one that does allow at
least +-10V output voltage swing, and my "boss" approved. The thing is
that I tried to find a replacement but couldn`t find one that`s
available here in Argentina.

It's probable that you can put the switching elsewhere than at the +/-10
V level, so you can use an ordinary switch.

To sum up what I`m asking for is a suitable IC, or a new idea on how to
make the circuit.

I've given you a new idea, but we need to know the exact gains you need.

 

[martin griffith]

On 14 Sep 2006 10:53:14 -0700, in sci.electronics.design

Hi, recently I`ve started to work (ad honorem, as a Technician) on a
Mechanical Engeneering Lab that performs destructive tests. They have
tasked me (I`m in the Electronic Instrumentation Department) to design
an electronic circuit that will replace an obsolete Voltage Gain Switch
circuit. The obsolete circuit uses relays to change the operational
amplifiers with different gains. Basically we are 2 guys, I`m an
Electronic Technician on the first year of electronic Engeneering and
who's in charge is an about to graduate Electronic Engeneer.

The requirements are:

+-10V output voltage swing
Low Frequency, from 0 to 200hz
Low Cost
Easy-to-find ICs (ICs with high Availability)
4 selectable gains controlled by two bits
Null-Offset
High Precision Gain

The Technician before me did this circuit with 4 Operational Amplifiers
with different gains and an analog switch CD4051 or CD4066. This
circuit did not meet the requirement of +-10V output voltage swing, so
I have to redesign it. The gains are predetermined, I don`t have the
actual values right here, but the maximum was 5000. My idea is to
change the analog switch with one that does allow at least +-10V output
voltage swing, and my "boss" approved. The thing is that I tried to
find a replacement but couldn`t find one that`s available here in
Argentina.

To sum up what I`m asking for is a suitable IC, or a new idea on how to
make the circuit. Of course ideas to improve the original design are
always welcome. I know about amplifier design, but on the Technician
level, that is, I don`t know any practical or modern way of doing it,
just what the theory says. As you can see the requirements are
standard, not very demanding (I guess), if there's anything you think I
should consider please tell me about it.

Thanks in advance.

What is the minimum gain required?

As others have said, 5000 is too much for a single opamp, so maybe a
fixed gain preamplifer, at the lowest gain may be adviseable


martin

 

[[email protected]]

Phil Hobbs ha escrito:

(snip)

What's wrong with the relays? For that job, relays aren't a dumb idea
at all. The analogue switches will have some nonlinearity and some
temperature coefficient of resistance, whereas relays are nearly
perfect--their only drawbacks are cost, which you don't care about for a
one-off, and slow switching speed, which you also don't care about.

Cheers,

Phil Hobbs

Yes you are right, but my task was to convert the circuit to
solid-state. It`s kind of a test. There's no way around that, plus
there's those old-fashioned switching noises, the bulkiness,ect.
Although I admire the ease of use of the relays


John Woodgate ha escrito:

That data is CRUCIAL for us to recommend a solution. 5000 is too much to
get reliably from one op-amp, even at 200 Hz.

The Gains are: 100, 500, 1000 and 5000
The other circuit has a pre-amplifier with a gain of 100, and the the
other four amplifiers with gains of 1, 5, 10 and 50 respectively. It
uses standard Op-Amps I don`t remember which model, all I know is that
there are no calculation's done about them, except for the
negative-feedback gain.

It's probable that you can put the switching elsewhere than at the +/-10
V level, so you can use an ordinary switch.

The overall output of the circuit must be able to swing to +-10V, it
was a requirement, I haven`t talked to my "Boss" about it, and probably
won`t until Monday. The input is from an LVPD transducer, I don`t know
much about those.

I've given you a new idea, but we need to know the exact gains you need.

Let me know if I missed something else.

Thanks.

 

[martin griffith]

[email protected] wrote:

The Gains are: 100, 500, 1000 and 5000
The other circuit has a pre-amplifier with a gain of 100, and the the
other four amplifiers with gains of 1, 5, 10 and 50 respectively. It
uses standard Op-Amps I don`t remember which model, all I know is that
there are no calculation's done about them, except for the
negative-feedback gain.

It's probable that you can put the switching elsewhere than at the +/-10
V level, so you can use an ordinary switch.

The overall output of the circuit must be able to swing to +-10V, it
was a requirement, I haven`t talked to my "Boss" about it, and probably
won`t until Monday. The input is from an LVPD transducer, I don`t know
much about those.

I've given you a new idea, but we need to know the exact gains you need.

Let me know if I missed something else.
[/QUOTE]
yep
http://www.pickeringrelay.com/

Relays can be very small, sealed, and offer much better performance
generally than fet type switches, but not as fast, and many companies
make them


martin

 

[John Woodgate]

The Gains are: 100, 500, 1000 and 5000
The other circuit has a pre-amplifier with a gain of 100, and the the
other four amplifiers with gains of 1, 5, 10 and 50 respectively. It
uses standard Op-Amps I don`t remember which model, all I know is that
there are no calculation's done about them, except for the
negative-feedback gain.

Doing it that way, no input signal must overload your high-gain
preamplifier. I would reduce that gain to 20, and follow it with four
op-amps as you have already. Then I would finish with an output stage
that has a gain of 5 and will produce +/- 10 V output. That way, your
switch never has to see more than 2 V.

That uses one more op-amp, but they are cheap. Almost all 'cooking'
op-amps will produce +/-10 V up to 200 Hz if you have +/- 15 V supplies
(maybe less) and a load on the output greater than 1 kohm. But avoid
LM324 and LM358, which need subtle treatment that will bother you.

 

[[email protected]]

martin griffith ha escrito:

The Gains are: 100, 500, 1000 and 5000
The other circuit has a pre-amplifier with a gain of 100, and the the
other four amplifiers with gains of 1, 5, 10 and 50 respectively. It
uses standard Op-Amps I don`t remember which model, all I know is that
there are no calculation's done about them, except for the
negative-feedback gain.


The overall output of the circuit must be able to swing to +-10V, it
was a requirement, I haven`t talked to my "Boss" about it, and probably
won`t until Monday. The input is from an LVPD transducer, I don`t know
much about those.


Let me know if I missed something else.

yep
http://www.pickeringrelay.com/

Relays can be very small, sealed, and offer much better performance
generally than fet type switches, but not as fast, and many companies
make them


martin[/QUOTE]

If you tell me that I can find those here in Argentina at an affordable
cost, you may have a point. Still I would have trouble convincing my
"Boss". It`s not a solution, but an alternative if no solution is to be
found, which I find hard to believe.

By the way they told they bought some instrumentation Amplifiers to
replace some old Amps they had (similar requirements), but they are
expensive (I`m talking about a working circuit not an IC). So yes it
can be done, and the cost is an issue.

 

[martin griffith]

On 14 Sep 2006 13:18:31 -0700, in sci.electronics.design

martin griffith ha escrito:


If you tell me that I can find those here in Argentina at an affordable
cost, you may have a point. Still I would have trouble convincing my
"Boss". It`s not a solution, but an alternative if no solution is to be
found, which I find hard to believe.

By the way they told they bought some instrumentation Amplifiers to
replace some old Amps they had (similar requirements), but they are
expensive (I`m talking about a working circuit not an IC). So yes it
can be done, and the cost is an issue.

Dont have a clue about Argentina's electronic distribution network,
what about online suppliers?

and ,yes, Cost is usually an issue


martin

 

[Phil Hobbs]

Phil Hobbs ha escrito:



Yes you are right, but my task was to convert the circuit to
solid-state. It`s kind of a test.

Ah, okay, a job security issue, gotcha. We can all understand that,
except for the old retired farts cluttering up the place.


I'd probably start with a gain-of-100 preamp, followed by a high
impedance passive attenuator, followed by a gain-of-50 output stage.
The input errors of the second stage won't be a problem, because the net
gain from the preamp input to the output of the attenuator will always
be at least 2. For an instrument, you probably want a nice quiet JFET
amp like an OPA656, or if your impedance levels are low, a LT1028 bipolar.

The voltage swing at the output of the attenuator will never be more
than 0.2V, which makes the linearity problem easier to deal with.
There'll be no bandwidth change with gain, because both amps are running
at constant gain.

I might use a voltage divider something like this:

0-----RRRRRRR------*--------*-------*-------0
| | |
30k R R R
R R R
7k50 R 3.33k R 610 R
R R R
R R R
| | |
0 0 0

2N7002 2N7002 2N7002

gnd gnd gnd

You could probably get away with some DMOS peripheral driver chip for
all 3 switches, because none of them will ever get further than 0.2V
from ground in normal operation, so none of the protection diodes will
turn on. The 2N7002 gets down to 2 or 3 ohms on resistance, with some
very small nonlinearity at the current levels we're talking about.
Their capacitances are in the 100-pF range at low voltage, giving a 3 dB
frequency of something like 18 kHz for three of them.

Cheers,

Phil Hobbs

 

[LT]

Leo,

The thing you need is an analog switch and op-amps that can handle
+/-10 volt swings as I read it.

I have used an Analog Devices, others make it, which is old, over 15
years, but still avialable. It is a ADG509 which is a dual 4 to 1
analog switch controlled with binary 2 bit TTL logic inputs. It can be
powered from +/- 15 V.

You would use with gain op-amp with the inputs having a resistor/pot
(pot to set exact gain) combinations driving neg input. Will need to
drive inverting buffer so output follows input. Could use with voltage
divider combination, one on each input and one on output of switch and
drive the + input of op-amp. Select the voltage divider combinations
to set the gain.

For +/- 10 volts will need to power all analog on +/-12 or more.

You could also use op-amp with this same analog switch selecting
different feed back resistors for the op-amp to adjust gain.

The ADG509 is wide band DC to over 5 MHz. Have used in video switchers
also.

Good luck to you and your co-worker about to graduate with EE degree.
I still remember the butter flys when I graduated in 1975.

73, ron, n9ee/r


You would drive

 

[Phil Hobbs]

Ah, okay, a job security issue, gotcha. We can all understand that,
except for the old retired farts cluttering up the place.


I'd probably start with a gain-of-100 preamp, followed by a high
impedance passive attenuator, followed by a gain-of-50 output stage. The
input errors of the second stage won't be a problem, because the net
gain from the preamp input to the output of the attenuator will always
be at least 2. For an instrument, you probably want a nice quiet JFET
amp like an OPA656, or if your impedance levels are low, a LT1028 bipolar.

The voltage swing at the output of the attenuator will never be more
than 0.2V, which makes the linearity problem easier to deal with.
There'll be no bandwidth change with gain, because both amps are running
at constant gain.

I might use a voltage divider something like this:

0-----RRRRRRR------*--------*-------*-------0
| | |
30k R R R
R R R
7k50 R 3.33k R 610 R
R R R
R R R
| | |
0 0 0

2N7002 2N7002 2N7002

gnd gnd gnd

You could probably get away with some DMOS peripheral driver chip for
all 3 switches, because none of them will ever get further than 0.2V
from ground in normal operation, so none of the protection diodes will
turn on. The 2N7002 gets down to 2 or 3 ohms on resistance, with some
very small nonlinearity at the current levels we're talking about. Their
capacitances are in the 100-pF range at low voltage, giving a 3 dB
frequency of something like 18 kHz for three of them.

Cheers,

Phil Hobbs

Oops. There *is* a bandwidth change with gain, because as soon as you
turn on one of the switches, the capacitance of the others ceases to
matter so much, and the bandwidth goes way up. Add a capacitor in
parallel with the feedback resistor of the second stage to drop the
bandwidth down to 1 kHz or so.

Cheers,

Phil Hobbs

 

[Michael A. Terrell]

Ah, okay, a job security issue, gotcha. We can all understand that,
except for the old retired farts cluttering up the place.

Be careful, Phil. "The old farts" hold the long term lease for this
newsgroup.



--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[[email protected]]

Ah, okay, a job security issue, gotcha. We can all understand that,
except for the old retired farts cluttering up the place.


I'd probably start with a gain-of-100 preamp, followed by a high
impedance passive attenuator, followed by a gain-of-50 output stage.
The input errors of the second stage won't be a problem, because the net
gain from the preamp input to the output of the attenuator will always
be at least 2. For an instrument, you probably want a nice quiet JFET
amp like an OPA656, or if your impedance levels are low, a LT1028 bipolar.

The LT1028 has been a favourite of mine for a very long time, but the
more recent Analog Devices AD797 is a little bit nicer.

http://www.analog.com/UploadedFiles/Data_Sheets/336938187AD797_e.pdf

The main difference seems to be in the output stage, which is a bit
less cranky than the LT1028's.

 

[Phil Hobbs]

The LT1028 has been a favourite of mine for a very long time, but the
more recent Analog Devices AD797 is a little bit nicer.

http://www.analog.com/UploadedFiles/Data_Sheets/336938187AD797_e.pdf

The main difference seems to be in the output stage, which is a bit
less cranky than the LT1028's.

Nice part, thanks.

Cheers,

Phil Hobbs

 

[David L. Jones]

Hi, recently I`ve started to work (ad honorem, as a Technician) on a
Mechanical Engeneering Lab that performs destructive tests. They have
tasked me (I`m in the Electronic Instrumentation Department) to design
an electronic circuit that will replace an obsolete Voltage Gain Switch
circuit. The obsolete circuit uses relays to change the operational
amplifiers with different gains. Basically we are 2 guys, I`m an
Electronic Technician on the first year of electronic Engeneering and
who's in charge is an about to graduate Electronic Engeneer.

The requirements are:

+-10V output voltage swing
Low Frequency, from 0 to 200hz
Low Cost
Easy-to-find ICs (ICs with high Availability)
4 selectable gains controlled by two bits
Null-Offset
High Precision Gain

The Technician before me did this circuit with 4 Operational Amplifiers
with different gains and an analog switch CD4051 or CD4066. This
circuit did not meet the requirement of +-10V output voltage swing, so
I have to redesign it. The gains are predetermined, I don`t have the
actual values right here, but the maximum was 5000. My idea is to
change the analog switch with one that does allow at least +-10V output
voltage swing, and my "boss" approved. The thing is that I tried to
find a replacement but couldn`t find one that`s available here in
Argentina.

To sum up what I`m asking for is a suitable IC, or a new idea on how to
make the circuit. Of course ideas to improve the original design are
always welcome. I know about amplifier design, but on the Technician
level, that is, I don`t know any practical or modern way of doing it,
just what the theory says. As you can see the requirements are
standard, not very demanding (I guess), if there's anything you think I
should consider please tell me about it.

Thanks in advance.

Use a high precision differential amplifier whose gain can be
controlled with a single resistor (0.1% or better depending on your
requirements). Use relays to switch the gain resistor. The AD620 is a
good starting point:
http://www.analog.com/en/prod/0,2877,AD620,00.html
datasheet:
http://www.analog.com/UploadedFiles/Data_Sheets/897653854AD620_g.pdf

The AD620 can do gains up to 10000, and should be able to do 200Hz at a
gain of 5000, but I don't think there is a Gain vs Freq curve above
1000 if memory serves me correctly.

Of course you'll have to watch your error budget very closely at such
high gains.

Dave

 

[[email protected]]

Phil Hobbs ha escrito:

Ah, okay, a job security issue, gotcha. We can all understand that,
except for the old retired farts cluttering up the place.


I'd probably start with a gain-of-100 preamp, followed by a high
impedance passive attenuator, followed by a gain-of-50 output stage.
The input errors of the second stage won't be a problem, because the net
gain from the preamp input to the output of the attenuator will always
be at least 2. For an instrument, you probably want a nice quiet JFET
amp like an OPA656, or if your impedance levels are low, a LT1028 bipolar.

The voltage swing at the output of the attenuator will never be more
than 0.2V, which makes the linearity problem easier to deal with.
There'll be no bandwidth change with gain, because both amps are running
at constant gain.

I might use a voltage divider something like this:

0-----RRRRRRR------*--------*-------*-------0
| | |
30k R R R
R R R
7k50 R 3.33k R 610 R
R R R
R R R
| | |
0 0 0

2N7002 2N7002 2N7002

gnd gnd gnd

You could probably get away with some DMOS peripheral driver chip for
all 3 switches, because none of them will ever get further than 0.2V
from ground in normal operation, so none of the protection diodes will
turn on. The 2N7002 gets down to 2 or 3 ohms on resistance, with some
very small nonlinearity at the current levels we're talking about.
Their capacitances are in the 100-pF range at low voltage, giving a 3 dB
frequency of something like 18 kHz for three of them.

Cheers,

Phil Hobbs

I`m going to answer the designs one by one.

About this one, I understood the general idea, which I believe is to
switch gains through the MOSFETs. But, I have no idea about the second
stage, the attenuator. First the signal gets amplified then attenuated,
the overall gain of the two stages being at least 2. Then it goes to
the third stage which has a fixed gain of 50 ? Which of the stages has
the selectable gain (voltage divider)? You probably should mention a
standard DMOS driver because I have never used one before.

It`s unlikely that I will use this design, but I am interested in how
it works, seems rather unconventional

 

[[email protected]]

LT ha escrito:

Leo,

The thing you need is an analog switch and op-amps that can handle
+/-10 volt swings as I read it.

I have used an Analog Devices, others make it, which is old, over 15
years, but still avialable. It is a ADG509 which is a dual 4 to 1
analog switch controlled with binary 2 bit TTL logic inputs. It can be
powered from +/- 15 V.

You would use with gain op-amp with the inputs having a resistor/pot
(pot to set exact gain) combinations driving neg input. Will need to
drive inverting buffer so output follows input. Could use with voltage
divider combination, one on each input and one on output of switch and
drive the + input of op-amp. Select the voltage divider combinations
to set the gain.

For +/- 10 volts will need to power all analog on +/-12 or more.

You could also use op-amp with this same analog switch selecting
different feed back resistors for the op-amp to adjust gain.

The ADG509 is wide band DC to over 5 MHz. Have used in video switchers
also.

Good luck to you and your co-worker about to graduate with EE degree.
I still remember the butter flys when I graduated in 1975.

73, ron, n9ee/r


You would drive

I`ve seen the ADG509 for sale here in Argentina, but I still don`t know
the cost. Seems like the way to go if it is affordable.

About the design, the guy before had the idea of switching the feedback
resistor, but my co-worker (the guy who's in charge of the department,
meaning he gets paid and I don`t) argued that the on resistance of the
switch would interfere with the presicion of the gain, but now that if
think about it, given a device it can be adjusted by trimpots only once
and should remain stable enough, but don`t know about that.

Thanks for the luck, I think I`ll be needing it much longer than he
does since I`ve just started and he's about to graduate

 

[[email protected]]

David L. Jones ha escrito:

Use a high precision differential amplifier whose gain can be
controlled with a single resistor (0.1% or better depending on your
requirements). Use relays to switch the gain resistor. The AD620 is a
good starting point:
http://www.analog.com/en/prod/0,2877,AD620,00.html
datasheet:
http://www.analog.com/UploadedFiles/Data_Sheets/897653854AD620_g.pdf

The AD620 can do gains up to 10000, and should be able to do 200Hz at a
gain of 5000, but I don't think there is a Gain vs Freq curve above
1000 if memory serves me correctly.

Of course you'll have to watch your error budget very closely at such
high gains.

Dave

Thanks Dave, the high-precision amplifier is a good idea, but I still
have that question about the feedback resistor switching.

 

[[email protected]]

John Woodgate ha escrito:

Doing it that way, no input signal must overload your high-gain
preamplifier. I would reduce that gain to 20, and follow it with four
op-amps as you have already. Then I would finish with an output stage
that has a gain of 5 and will produce +/- 10 V output. That way, your
switch never has to see more than 2 V.

That uses one more op-amp, but they are cheap. Almost all 'cooking'
op-amps will produce +/-10 V up to 200 Hz if you have +/- 15 V supplies
(maybe less) and a load on the output greater than 1 kohm. But avoid
LM324 and LM358, which need subtle treatment that will bother you.

The Op-Amps I`m gonna stick with are OP07s, unless there is a major
problem with them. I like the idea that the switch doesn`t have to be
the final stage, somehow I took that idea off my mind. Still I have a
problem using 4051s or 4066 because none of those would allow +-12V
power supplys, so I will have to lower the voltage to power them, I
will probably use some zenner diode.