low-cost high-voltage electrometer amplifier

[Winfield Hill]

Here's a nice example of a circuit that pulls itself
up by its bootstraps - a high-voltage opamp follower
featuring electrometer-style high input impedances.

My design is meant to replace expensive high-voltage
opamps, like the PA97 offered by Apex, with low-cost
superior-performance low-voltage opamps, like those
offered by Analog Devices (Stephan Goldstein should
enjoy this one). My goal is for an amplifier with
an offset voltage under 65uV and less than 1pA input
current, yet operating over a 430-volt signal range.

+420
_|
IN ___| \___ OUT - within
<1pA |__/ 65uV of input
|
-15

For example, Analog Devices' ad8605 is a quiet (8.5nV)
fast (10MHz, 5V/us) sot-23 opamp that meets two out
of three of my specs, and yet only costs $1, qty 100.
http://www.analog.com/en/prod/0,2877,AD8605,00.html

The ad8605 fails the 400V spec, of course. Here's the
basic idea for my solution, which features two elegant
450V Supertex depletion-mode MOSFETs. This is just
the basic idea; my full circuit takes 28 to 35 parts.

+420
|
|--'
,----||<-,
input | |--+ dn3145
protection _____|________|
\__ _|_ | |
IN --|__|---|+ \ | _|_,
| >--' /_\ 5.1V
,--|-__/ |
|____|______________|____ OUT
ad805 |
|
1.5mA
sink

Well, whaddya think?

 

[John Larkin]

Here's a nice example of a circuit that pulls itself
up by its bootstraps - a high-voltage opamp follower
featuring electrometer-style high input impedances.

My design is meant to replace expensive high-voltage
opamps, like the PA97 offered by Apex, with low-cost
superior-performance low-voltage opamps, like those
offered by Analog Devices (Stephan Goldstein should
enjoy this one). My goal is for an amplifier with
an offset voltage under 65uV and less than 1pA input
current, yet operating over a 430-volt signal range.

+420
_|
IN ___| \___ OUT - within
<1pA |__/ 65uV of input
|
-15

For example, Analog Devices' ad8605 is a quiet (8.5nV)
fast (10MHz, 5V/us) sot-23 opamp that meets two out
of three of my specs, and yet only costs $1, qty 100.
http://www.analog.com/en/prod/0,2877,AD8605,00.html

The ad8605 fails the 400V spec, of course. Here's the
basic idea for my solution, which features two elegant
450V Supertex depletion-mode MOSFETs. This is just
the basic idea; my full circuit takes 28 to 35 parts.

+420
|
|--'
,----||<-,
input | |--+ dn3145
protection _____|________|
\__ _|_ | |
IN --|__|---|+ \ | _|_,
| >--' /_\ 5.1V
,--|-__/ |
|____|______________|____ OUT
ad805 |
|
1.5mA
sink

Well, whaddya think?

Cute. I've seen opamps whose rails tracked the opamp Vout, intended to
give huge cmrr's.

Post the full circuit!

John

 

[Jan Panteltje]

Here's a nice example of a circuit that pulls itself
up by its bootstraps - a high-voltage opamp follower
featuring electrometer-style high input impedances.

My design is meant to replace expensive high-voltage
opamps, like the PA97 offered by Apex, with low-cost
superior-performance low-voltage opamps, like those
offered by Analog Devices (Stephan Goldstein should
enjoy this one). My goal is for an amplifier with
an offset voltage under 65uV and less than 1pA input
current, yet operating over a 430-volt signal range.

+420
_|
IN ___| \___ OUT - within
<1pA |__/ 65uV of input
|
-15

For example, Analog Devices' ad8605 is a quiet (8.5nV)
fast (10MHz, 5V/us) sot-23 opamp that meets two out
of three of my specs, and yet only costs $1, qty 100.
http://www.analog.com/en/prod/0,2877,AD8605,00.html

The ad8605 fails the 400V spec, of course. Here's the
basic idea for my solution, which features two elegant
450V Supertex depletion-mode MOSFETs. This is just
the basic idea; my full circuit takes 28 to 35 parts.

+420
|
|--'
,----||<-,
input | |--+ dn3145
protection _____|________|
\__ _|_ | |
IN --|__|---|+ \ | _|_,
| >--' /_\ 5.1V
,--|-__/ |
|____|______________|____ OUT
ad805 |
|
1.5mA
sink

Well, whaddya think?

I am curious about your 'input protection',
would there not be a very short input current spike if you
applied a step from 0 to +400V ?
That could cause problenms in some circuit.

 

[Rene Tschaggelar]

Here's a nice example of a circuit that pulls itself
up by its bootstraps - a high-voltage opamp follower
featuring electrometer-style high input impedances.

My design is meant to replace expensive high-voltage
opamps, like the PA97 offered by Apex, with low-cost
superior-performance low-voltage opamps, like those
offered by Analog Devices (Stephan Goldstein should
enjoy this one). My goal is for an amplifier with
an offset voltage under 65uV and less than 1pA input
current, yet operating over a 430-volt signal range.

+420
_|
IN ___| \___ OUT - within
<1pA |__/ 65uV of input
|
-15
[snip]

Well, whaddya think?

Win,
I'm interested. Considering the cost of an
APEX part, there is some slack in building
the function from scratch with 35 parts.

Rene

 

[John Larkin]

Here's a nice example of a circuit that pulls itself
up by its bootstraps - a high-voltage opamp follower
featuring electrometer-style high input impedances.

My design is meant to replace expensive high-voltage
opamps, like the PA97 offered by Apex, with low-cost
superior-performance low-voltage opamps, like those
offered by Analog Devices (Stephan Goldstein should
enjoy this one). My goal is for an amplifier with
an offset voltage under 65uV and less than 1pA input
current, yet operating over a 430-volt signal range.

+420
_|
IN ___| \___ OUT - within
<1pA |__/ 65uV of input
|
-15
[snip]

Well, whaddya think?

Win,
I'm interested. Considering the cost of an
APEX part, there is some slack in building
the function from scratch with 35 parts.

Rene

OK, what am I missing?

http://s2.supload.com/free/WinsHVamp.JPG/view/


John

 

[Winfield Hill]

I am curious about your 'input protection',
would there not be a very short input current
spike if you applied a step from 0 to +400V ?
That could cause problems in some circuit.

Yes indeed. For example, with its infinite Zin and
vanishingly-low input current, if a probe is used to
measure +400 volts, after removal the amplifier will
simply stay there. Then if the probe is touched to
ground, the amplifier's input will experience an
instantaneous -400-volt spike.

The easy fix is a pair of back-to-back diodes at
the input to the circuit's "supply-ground" output,
along with some current limiting input resistance.
I chose two 1k resistors in series at the input,
along with one more between the diode node and the
opamp to limit its current during the fault. The
low value was chosen to preserve the opamp's 8.5nV
voltage noise. But 2k input resistance allows for
a 200mA fault current, so I added a depletion-mode
MOSFET circuit to further limit the fault current
to about +/- 0.5 to 1mA.

I used my favorite Supertex LND150 or LND250 parts.
The protection diodes are low-leakage low-capacitance
BJT base-collector junctions.

500V protection
,-diodes-,
1k | | 1k
o--/\/\--, ,---+--/\/\--+---, ,--/\/\--+--/\/\-- opamp
_|__|_ | | _|__|_ |
---, | | ,--- +--|>|--,
|___| _______| | '--|<|--+--- out
|____________|

This protection circuit can withstand a continuous
input connection to +400 volts, while the circuit's
high-voltage power source is off.

I also added parts to the rest of the circuit to
protect the MOSFETs, etc., during the fast slewing.

 

[Winfield Hill]

Here's a nice example of a circuit that pulls itself
up by its bootstraps - a high-voltage opamp follower
featuring electrometer-style high input impedances.

My design is meant to replace expensive high-voltage
opamps, like the PA97 offered by Apex, with low-cost
superior-performance low-voltage opamps, like those
offered by Analog Devices (Stephan Goldstein should
enjoy this one). My goal is for an amplifier with
an offset voltage under 65uV and less than 1pA input
current, yet operating over a 430-volt signal range.

+420
_|
IN ___| \___ OUT - within
<1pA |__/ 65uV of input
|
-15
[snip]

Well, whaddya think?

Win,
I'm interested. Considering the cost of an
APEX part, there is some slack in building
the function from scratch with 35 parts.

Well, in its favor, the APEX does have lots of
features mine lacks, such as high output drive.
But they would make for expensive HV-voltmeter
high-impedance voltage followers. We needed
two inputs for a differential measurement, so
I decided to design this circuit. There are
several applications where it can be used.

 

[John Larkin]

Here's a nice example of a circuit that pulls itself
up by its bootstraps - a high-voltage opamp follower
featuring electrometer-style high input impedances.

My design is meant to replace expensive high-voltage
opamps, like the PA97 offered by Apex, with low-cost
superior-performance low-voltage opamps, like those
offered by Analog Devices (Stephan Goldstein should
enjoy this one). My goal is for an amplifier with
an offset voltage under 65uV and less than 1pA input
current, yet operating over a 430-volt signal range.

+420
_|
IN ___| \___ OUT - within
<1pA |__/ 65uV of input
|
-15
[snip]

Well, whaddya think?

Win,
I'm interested. Considering the cost of an
APEX part, there is some slack in building
the function from scratch with 35 parts.

Well, in its favor, the APEX does have lots of
features mine lacks, such as high output drive.
But they would make for expensive HV-voltmeter
high-impedance voltage followers. We needed
two inputs for a differential measurement, so
I decided to design this circuit. There are
several applications where it can be used.

You could also do a flying-capacitor thing, with a dpdt relay, if
static measurements were enough. It could report the differential
input almost 1:1, or you could dump into a bigger cap and get a
voltage-divider effect.

John

 

[[email protected]]

Here's a nice example of a circuit that pulls itself
up by its bootstraps - a high-voltage opamp follower
featuring electrometer-style high input impedances.

My design is meant to replace expensive high-voltage
opamps, like the PA97 offered by Apex, with low-cost
superior-performance low-voltage opamps, like those
offered by Analog Devices (Stephan Goldstein should
enjoy this one). My goal is for an amplifier with
an offset voltage under 65uV and less than 1pA input
current, yet operating over a 430-volt signal range.

+420
_|
IN ___| \___ OUT - within
<1pA |__/ 65uV of input
|
-15

For example, Analog Devices' ad8605 is a quiet (8.5nV)
fast (10MHz, 5V/us) sot-23 opamp that meets two out
of three of my specs, and yet only costs $1, qty 100.
http://www.analog.com/en/prod/0,2877,AD8605,00.html

The ad8605 fails the 400V spec, of course. Here's the
basic idea for my solution, which features two elegant
450V Supertex depletion-mode MOSFETs. This is just
the basic idea; my full circuit takes 28 to 35 parts.

+420
|
|--'
,----||<-,
input | |--+ dn3145
protection _____|________|
\__ _|_ | |
IN --|__|---|+ \ | _|_,
| >--' /_\ 5.1V
,--|-__/ |
|____|______________|____ OUT
ad805 |
|
1.5mA
sink

Well, whaddya think?

You could allways get a service manual for a Tek 7a13 plugin and look
how they did it.

 

[John Larkin]

You could allways get a service manual for a Tek 7a13 plugin and look
how they did it.

Or a type Z.

John

 

[Winfield]

Or a type Z.

What, tubes? That's going backwards, not forwards!

 

[Winfield]

Winfield Hill wrote:

Here's a nice example of a circuit that pulls itself
up by its bootstraps - a high-voltage opamp follower
featuring electrometer-style high input impedances.
My design is meant to replace expensive high-voltage
opamps, like the PA97 offered by Apex, with low-cost
superior-performance low-voltage opamps, like those
offered by Analog Devices (Stephan Goldstein should
enjoy this one). My goal is for an amplifier with
an offset voltage under 65uV and less than 1pA input
current, yet operating over a 430-volt signal range.
+420
_|
IN ___| \___ OUT - within
<1pA |__/ 65uV of input
|
-15

[snip]

Well, whaddya think?

Win,
I'm interested. Considering the cost of an
APEX part, there is some slack in building
the function from scratch with 35 parts.

OK, what am I missing?
http://s2.supload.com/free/WinsHVamp.JPG/view/

My input current-limiting circuit, transient
protection for the MOSFETs, a small isolated
capacitor to absorb fault charge, a protected
guard circuit, and series resistors to limit
the fault currents if a MOSFET fails.

You have an extra zener with cap, unless you
want to add an active PMOS pulldown circuit
to increase negative slew-rate capability.
That's my next version and pushes the parts
count up to 38.

 

[Rene Tschaggelar]

Winfield Hill wrote:

Here's a nice example of a circuit that pulls itself
up by its bootstraps - a high-voltage opamp follower
featuring electrometer-style high input impedances.

My design is meant to replace expensive high-voltage
opamps, like the PA97 offered by Apex, with low-cost
superior-performance low-voltage opamps, like those
offered by Analog Devices (Stephan Goldstein should
enjoy this one). My goal is for an amplifier with
an offset voltage under 65uV and less than 1pA input
current, yet operating over a 430-volt signal range.

+420
_|
IN ___| \___ OUT - within
<1pA |__/ 65uV of input
|
-15

[snip]


Well, whaddya think?

Win,
I'm interested. Considering the cost of an
APEX part, there is some slack in building
the function from scratch with 35 parts.

Well, in its favor, the APEX does have lots of
features mine lacks, such as high output drive.
But they would make for expensive HV-voltmeter
high-impedance voltage followers. We needed
two inputs for a differential measurement, so
I decided to design this circuit. There are
several applications where it can be used.

You could also do a flying-capacitor thing, with a dpdt relay, if
static measurements were enough. It could report the differential
input almost 1:1, or you could dump into a bigger cap and get a
voltage-divider effect.

John,
and the switches for the high voltage being ?
There are several conditions to it. Such as
low leakage, low charge injection. Win was
talking about an electrometer input, meaning
there cannot 1uF at the input.

Rene

 

[Rene Tschaggelar]

Winfield Hill wrote:

Here's a nice example of a circuit that pulls itself
up by its bootstraps - a high-voltage opamp follower
featuring electrometer-style high input impedances.

My design is meant to replace expensive high-voltage
opamps, like the PA97 offered by Apex, with low-cost
superior-performance low-voltage opamps, like those
offered by Analog Devices (Stephan Goldstein should
enjoy this one). My goal is for an amplifier with
an offset voltage under 65uV and less than 1pA input
current, yet operating over a 430-volt signal range.

+420
_|
IN ___| \___ OUT - within
<1pA |__/ 65uV of input
|
-15

[snip]

Well, whaddya think?

Win,
I'm interested. Considering the cost of an
APEX part, there is some slack in building
the function from scratch with 35 parts.

OK, what am I missing?

http://s2.supload.com/free/WinsHVamp.JPG/view/

The depletion Fets are single sourced ? I only
heard about one manufacturer once, and even
forgot about it. I'd prefer enhancement MosFets.
A schematic is one thing and some estmations,
simulations or measurements are another.
What are expectable slew rates ?

Rene

 

[Jan Panteltje]

Yes indeed. For example, with its infinite Zin and
vanishingly-low input current, if a probe is used to
measure +400 volts, after removal the amplifier will
simply stay there. Then if the probe is touched to
ground, the amplifier's input will experience an
instantaneous -400-volt spike.

The easy fix is a pair of back-to-back diodes at
the input to the circuit's "supply-ground" output,
along with some current limiting input resistance.
I chose two 1k resistors in series at the input,
along with one more between the diode node and the
opamp to limit its current during the fault. The
low value was chosen to preserve the opamp's 8.5nV
voltage noise. But 2k input resistance allows for
a 200mA fault current, so I added a depletion-mode
MOSFET circuit to further limit the fault current
to about +/- 0.5 to 1mA.

I used my favorite Supertex LND150 or LND250 parts.
The protection diodes are low-leakage low-capacitance
BJT base-collector junctions.

500V protection
,-diodes-,
1k | | 1k
o--/\/\--, ,---+--/\/\--+---, ,--/\/\--+--/\/\-- opamp
_|__|_ | | _|__|_ |
---, | | ,--- +--|>|--,
|___| _______| | '--|<|--+--- out
|____________|

This protection circuit can withstand a continuous
input connection to +400 volts, while the circuit's
high-voltage power source is off.

I also added parts to the rest of the circuit to
protect the MOSFETs, etc., during the fast slewing.

Beautiful, this one I will keep on the harddisk
Thank you.

 

[Winfield]

The depletion Fets are single sourced? I only
heard about one manufacturer once, and even
forgot about it.

There are at least three manufacturers, although
Supertex is the most prolific and committed one.

I'd prefer enhancement MosFets.

Unh Huh. Be my guest trying to create a simple
effective circuit like this with an enhancement-
mode MOSFET. You need ugly things, like a pullup
resistor that has to provide sufficient current
over a two-decade range of signal voltages, or a
HV current source (which is best made using a
depletion-mode transistor), etc. I've done it,
multiple times, but it's a true pleasure to be
able to use depletion-mode instead. Supertex is
a reliable manufacturer, and now that Mouser is
stocking their parts there's little excuse to
avoid them. We all work with sole-source parts
all the time. Just maintain a good inventory.

A schematic is one thing and some estimations,
simulations or measurements are another.

What! You question the operational efficacy of my
circuits? Actually, this one has been reduced
to elegant practice, with a nice SMD-component PCB.

What are expectable slew rates?

Pullup slew rates are very fast, but in my simple
class-A circuit (again below), the fixed pulldown
current source is a limitation, against the combined
Coss of the pullup + pulldown MOSFETs, plus any load
capacitance, such as a triaxial guard cable, etc.

+420
|
|--'
,----||<-,
input | |--+ dn3145
protection _____|________|
\__ _|_ | |
IN --|__|---|+ \ | _|_,
| >--' /_\ 5.1V
,--|-__/ |
|____|______________|____ OUT
ad805 |
|
1.5mA
sink

If the load is just a resistive divider, the circuit
can be reasonably fast. Supertex dn3145 and dn2540
are candidate MOSFETs, with a Coss of about 12pF at
25V, so let's calculate for C = 25pF (for the pullup
plus pulldown CS mosfets). dV/dt = i/C = 60V/us,
which means it takes 6.6us to slew 400 volts, and
can follow a 50kHz 400Vp-p sine wave.

My full circuit, 2nd rev, adds a p-channel pulldown
MOSFET, which I expect to double the negative slew rate
and maintain this speed into a triax guard capacitance.

 

[John Larkin]

There are at least three manufacturers, although
Supertex is the most prolific and committed one.


Unh Huh. Be my guest trying to create a simple
effective circuit like this with an enhancement-
mode MOSFET. You need ugly things, like a pullup
resistor that has to provide sufficient current
over a two-decade range of signal voltages, or a
HV current source (which is best made using a
depletion-mode transistor), etc.

Sometimes pv optoisolators can be handy in situations like this.

I've done it,
multiple times, but it's a true pleasure to be
able to use depletion-mode instead. Supertex is
a reliable manufacturer, and now that Mouser is
stocking their parts there's little excuse to
avoid them. We all work with sole-source parts
all the time. Just maintain a good inventory.

Nowadays, the idea of having 2 and 3 sources for everything just isn't
practical.

John

 

[John Larkin]

Winfield Hill wrote:


Here's a nice example of a circuit that pulls itself
up by its bootstraps - a high-voltage opamp follower
featuring electrometer-style high input impedances.

My design is meant to replace expensive high-voltage
opamps, like the PA97 offered by Apex, with low-cost
superior-performance low-voltage opamps, like those
offered by Analog Devices (Stephan Goldstein should
enjoy this one). My goal is for an amplifier with
an offset voltage under 65uV and less than 1pA input
current, yet operating over a 430-volt signal range.

+420
_|
IN ___| \___ OUT - within
<1pA |__/ 65uV of input
|
-15


[snip]

Well, whaddya think?

Win,
I'm interested. Considering the cost of an
APEX part, there is some slack in building
the function from scratch with 35 parts.

OK, what am I missing?

http://s2.supload.com/free/WinsHVamp.JPG/view/

The depletion Fets are single sourced ? I only
heard about one manufacturer once, and even
forgot about it. I'd prefer enhancement MosFets.

You can do some cool stuff with depletion fets, like very nice current
limiters. We've never had a problem getting the Supertex parts.

A schematic is one thing and some estmations,
simulations or measurements are another.

Certainly. I'd do all that if I had a reason to, and a target spec to
meet. At present, I don't have either.

What are expectable slew rates ?

You can do that math as well as I can.

Do you always rain on ideas?

John

 

[John Larkin]

Winfield Hill wrote:

Here's a nice example of a circuit that pulls itself
up by its bootstraps - a high-voltage opamp follower
featuring electrometer-style high input impedances.

My design is meant to replace expensive high-voltage
opamps, like the PA97 offered by Apex, with low-cost
superior-performance low-voltage opamps, like those
offered by Analog Devices (Stephan Goldstein should
enjoy this one). My goal is for an amplifier with
an offset voltage under 65uV and less than 1pA input
current, yet operating over a 430-volt signal range.

+420
_|
IN ___| \___ OUT - within
<1pA |__/ 65uV of input
|
-15

[snip]


Well, whaddya think?

Win,
I'm interested. Considering the cost of an
APEX part, there is some slack in building
the function from scratch with 35 parts.

Well, in its favor, the APEX does have lots of
features mine lacks, such as high output drive.
But they would make for expensive HV-voltmeter
high-impedance voltage followers. We needed
two inputs for a differential measurement, so
I decided to design this circuit. There are
several applications where it can be used.

You could also do a flying-capacitor thing, with a dpdt relay, if
static measurements were enough. It could report the differential
input almost 1:1, or you could dump into a bigger cap and get a
voltage-divider effect.

John,
and the switches for the high voltage being ?
There are several conditions to it. Such as
low leakage, low charge injection. Win was
talking about an electrometer input, meaning
there cannot 1uF at the input.

Rene

Some people can riff on ideas, and some people stamp them down as soon
as they appear.

John

 

[Rene Tschaggelar]

There are at least three manufacturers, although
Supertex is the most prolific and committed one.

Actually the preferrence was more towards something
buyable, not against depletion as itself.

Unh Huh. Be my guest trying to create a simple
effective circuit like this with an enhancement-
mode MOSFET. You need ugly things, like a pullup
resistor that has to provide sufficient current
over a two-decade range of signal voltages, or a
HV current source (which is best made using a
depletion-mode transistor), etc. I've done it,
multiple times, but it's a true pleasure to be
able to use depletion-mode instead. Supertex is
a reliable manufacturer, and now that Mouser is
stocking their parts there's little excuse to
avoid them. We all work with sole-source parts
all the time. Just maintain a good inventory.

Supertex ist having a strange policy. Their minimum
order quantity apparently is a wheel of 1k pieces.
Yes, they do samples and can be asked for lower
quantities, but with our distributor in between
they are somewhat unwieldy. Perhaps I should look
at Mouser, which haven't ordered yet from.

What! You question the operational efficacy of my
circuits? Actually, this one has been reduced
to elegant practice, with a nice SMD-component PCB.

No offense intended

Pullup slew rates are very fast, but in my simple
class-A circuit (again below), the fixed pulldown
current source is a limitation, against the combined
Coss of the pullup + pulldown MOSFETs, plus any load
capacitance, such as a triaxial guard cable, etc.

+420
|
|--'
,----||<-,
input | |--+ dn3145
protection _____|________|
\__ _|_ | |
IN --|__|---|+ \ | _|_,
| >--' /_\ 5.1V
,--|-__/ |
|____|______________|____ OUT
ad805 |
|
1.5mA
sink

If the load is just a resistive divider, the circuit
can be reasonably fast. Supertex dn3145 and dn2540
are candidate MOSFETs, with a Coss of about 12pF at
25V, so let's calculate for C = 25pF (for the pullup
plus pulldown CS mosfets). dV/dt = i/C = 60V/us,
which means it takes 6.6us to slew 400 volts, and
can follow a 50kHz 400Vp-p sine wave.

That sounds fantastic

My full circuit, 2nd rev, adds a p-channel pulldown
MOSFET, which I expect to double the negative slew rate
and maintain this speed into a triax guard capacitance.

Rene