Supply dependent voltage reference?

[MRW]

Hello once again!

I'm reading about Texas Instrument's Single Supply Opamp Circuits
reference guide. I was wondering if there is a way to create a
half-supply voltage reference that is completely dependent on the input
supply voltage.

The reason why I'm asking is because if the supply voltage drops, then
I want the voltage reference to also take this into account. For
example, if I start at 5V, then my initial half-supply voltage would be
2.5V. But in time if the supply voltage drops to 4.8V, then I want the
half-supply to automatically adjust to 2.4V.

Any recommendations?

Thanks!

 

[PeteS]

Hello once again!

I'm reading about Texas Instrument's Single Supply Opamp Circuits
reference guide. I was wondering if there is a way to create a
half-supply voltage reference that is completely dependent on the input
supply voltage.

The reason why I'm asking is because if the supply voltage drops, then
I want the voltage reference to also take this into account. For
example, if I start at 5V, then my initial half-supply voltage would be
2.5V. But in time if the supply voltage drops to 4.8V, then I want the
half-supply to automatically adjust to 2.4V.

Any recommendations?

Thanks!

A voltage divider with equal resistors from the supply you want to
reference from will do it. If you need low impedance, then use this as
an input to a voltage follower, and use the output as your reference, or
(for occasional transients only) put a 0.1uF cap across the resistor to
ground.

Cheers

PeteS

 

[Eeyore]

Hello once again!

I'm reading about Texas Instrument's Single Supply Opamp Circuits
reference guide. I was wondering if there is a way to create a
half-supply voltage reference that is completely dependent on the input
supply voltage.

The reason why I'm asking is because if the supply voltage drops, then
I want the voltage reference to also take this into account. For
example, if I start at 5V, then my initial half-supply voltage would be
2.5V. But in time if the supply voltage drops to 4.8V, then I want the
half-supply to automatically adjust to 2.4V.

Any recommendations?

That's the way it always used to be done.

A potential divider followed by an op-amp is how I've done it.

You may also want to provide some supply ripple filtering on the 1/2 Vsupply
reference too.

Graham

 

[MRW]

Thanks all!

I was actually thinking about the voltage divider setup, too. But it
didn't occur to me that can connect the divider output to a voltage
follower opamp. Initially, I thought that using the voltage divider
alone may pose some loading issues if I'm pulling too much current. Is
this a right thought?

 

[PeteS]

Thanks all!

I was actually thinking about the voltage divider setup, too. But it
didn't occur to me that can connect the divider output to a voltage
follower opamp. Initially, I thought that using the voltage divider
alone may pose some loading issues if I'm pulling too much current. Is
this a right thought?

You are correct. Keep in mind that any loading (by taking current)
presents an effective resistance to ground inversely proportional to the
loading.

One thing; unless you are using an opamp designed for it, do NOT put a
large amount (0.1uF is large) of capacitance on the output of the
follower; it will probably oscillate due to loop instability.

Cheers

PeteS

 

[MRW]

Thanks, PeteS!

You are correct. Keep in mind that any loading (by taking current)
presents an effective resistance to ground inversely proportional to the
loading.

Ahh.. It took me a few minutes to grasp this, but now I see it makes
perfect sense.

One thing; unless you are using an opamp designed for it, do NOT put a
large amount (0.1uF is large) of capacitance on the output of the
follower; it will probably oscillate due to loop instability.

Loop instability is still somewhat new to me.

Thanks!

 

[MRW]

Another question popped out regarding this topic. If I were planning on
using the voltage divider & voltage follower setup as my half-supply
source, what op amp parameters should I check? Will this have a similar
fan out characteristics that CMOS or TTL devices have?

 

[John Popelish]

Thanks, PeteS!


Ahh.. It took me a few minutes to grasp this, but now I see it makes
perfect sense.



Loop instability is still somewhat new to me.

An opamp in a negative feedback loop tries to produce an
output that makes its two inputs have the same voltage. If
the output is loaded by a lot of capacitance, changing the
output voltage requires that the output drive a large
current into the capacitor to charge it up. This can crank
the output stage all the way to current limit conditions,
which take a while to recover, once the correct voltage is
achieved. But during that recovery, the cap is charged past
the correct voltage, and the opamp has to suck current out
pf the capacitor to get its voltage lowered back to the
correct value. But if this process also hits the limit
current, the voltage will overshoot that way, also, and the
process repeats... an oscillation.

 

[PeteS]

An opamp in a negative feedback loop tries to produce an output that
makes its two inputs have the same voltage. If the output is loaded by
a lot of capacitance, changing the output voltage requires that the
output drive a large current into the capacitor to charge it up. This
can crank the output stage all the way to current limit conditions,
which take a while to recover, once the correct voltage is achieved.
But during that recovery, the cap is charged past the correct voltage,
and the opamp has to suck current out pf the capacitor to get its
voltage lowered back to the correct value. But if this process also
hits the limit current, the voltage will overshoot that way, also, and
the process repeats... an oscillation.

Well, that's one way of looking at it.

An op amp, as opposed to a comparator, has an RC pole (which induces
other things) internally, and quite deliberately, to roll the open loop
gain off.

Now that pole (sometimes known as the dominant pole) will have a -3dB
point at perhaps 10Hz (or below). At this point, it will have induced
-45 degrees of phase for a voltage style device. At 1 decade further
(100Hz in our simplistic example), it will be at 90 degrees and will
stay there.

Add another -90 degrees and what was negative feedback becomes positive
feedback. If that holds at the unity gain point, then you have an
oscillator.

This is a _very_ simplistic look at such things; a typical amp has
multiple poles and zeroes, but the datasheet usually has guidance on the
maximum output capacitance.

Cheers

PeteS

 

[Michael Black]

Thanks all!

I was actually thinking about the voltage divider setup, too. But it
didn't occur to me that can connect the divider output to a voltage
follower opamp. Initially, I thought that using the voltage divider
alone may pose some loading issues if I'm pulling too much current. Is
this a right thought?

If the current is higher, you used lower value (but still equal) resistors.
It places more of a load on the power supply, but it takes more load to
affect it.

Michael

 

[jasen]

The reason why I'm asking is because if the supply voltage drops, then
I want the voltage reference to also take this into account. For
example, if I start at 5V, then my initial half-supply voltage would be
2.5V. But in time if the supply voltage drops to 4.8V, then I want the
half-supply to automatically adjust to 2.4V.

use a voltage divider, possibly several.

 

[MRW]

This is a _very_ simplistic look at such things; a typical amp has
multiple poles and zeroes, but the datasheet usually has guidance on the
maximum output capacitance.

Using this datasheet guide, how would I be able to calculate or
estimate the maximum capacitance presented by other opamps being driven
by the half-supply source (voltage divider + opamp voltage follower
configuration)? Is there a datasheet parameter that will tell me this?

Also, after looking at the Single Supply circuit guide even more, I
noticed that the author mentions the following about the voltage
divider + voltage follower setup: "...but its performance deteriorates
at low frequencies."

Why is that?

Here is a link to the PDF:
http://focus.ti.com/lit/an/sloa058/sloa058.pdf

Thanks!

 

[PeteS]

Using this datasheet guide, how would I be able to calculate or
estimate the maximum capacitance presented by other opamps being driven
by the half-supply source (voltage divider + opamp voltage follower
configuration)? Is there a datasheet parameter that will tell me this?

The input capacitance of most active devices is specified in their
datasheets. You also need to add the track / wiring capacitance, but
most amps will be ok for any reasonable load. Note that not all
amplifiers are stable at unity gain. This is covered in most texts on
the subject.
If you haven't read the underlying theory of feedback, now would be a
good time to start

Also, after looking at the Single Supply circuit guide even more, I
noticed that the author mentions the following about the voltage
divider + voltage follower setup: "...but its performance deteriorates
at low frequencies."

Why is that?

As you approach DC, the output impedance of the amp approaches it's
output resistance, which is somewhat higher than the dynamic impedance
(such as step response). The output resistance of the amp will typically
be a few 10s of ohms, the impedance (in terms of step response) a few
milli ohms.
Incidentally, this circuit would benefit from a feedback resistor (see
my comments in the other thread) of about 50k rather than a straight
wire; that would miminise input current offsets.

Here is a link to the PDF:
http://focus.ti.com/lit/an/sloa058/sloa058.pdf

Thanks!

Cheers

PeteS