General > General Technical Chat
Low ESR Output Capacitor for REF5010 and REF5025
EPAIII:
A quick search did not yield any 300 Volt op amps. At least not such chips. Perhaps a discrete circuit? Or an op amp with a Voltage boost output circuit?
Any suggestions where I should look?
--- Quote from: tom66 on July 12, 2022, 10:00:16 am ---Why not just use precision resistors and an op-amp and power electronics circuit (or just a high-voltage opamp)? That way you only need one reference.
--- End quote ---
tom66:
This is available:
https://www.top-electronics.com/en/power-op-amp-900v-500v-us-100ma
Though its parameters are probably somewhat compromised given it is monolithic, and it is not cheap.
An alternative approach would be to build a low voltage opamp circuit with a feedback resistor network (which can be precisely set using e.g. 0.1% tolerance parts matched to each other) and either a DAC/PWM/potentiometer to set the feedback voltage from your precision reference.
The opamp would drive a form of high voltage amplifier. A common approach is two common emitter circuits in series, with high voltage NPNs. This adds phase delay to the circuit, so you need to compensate the opamp appropriately.
However complex this sounds, I doubt it is more complex than tying 30 odd Vref's in series with each other!
David Hess:
--- Quote from: tom66 on July 12, 2022, 10:00:16 am ---Why not just use precision resistors and an op-amp and power electronics circuit (or just a high-voltage opamp)? That way you only need one reference.
--- End quote ---
That is possible but presents calibration issues compared to stacked references.
Stacked references can be checked individually at low voltage. A precision high voltage amplifier depends on its high voltage feedback divider for an accurate output, and checking the accuracy of a high voltage divider is not easy. A low voltage test of the divider is not sufficient because at high voltages, the voltage coefficient of resistance becomes a factor and that can only be tested at high voltage.
The issues with a high voltage feedback divider can be overcome but require specific attention. What I might do is grade a bunch of the same resistor for precision, and then stack them to make the divider. Since all of the resistors will have the same voltage across them, the voltage coefficient of resistance will be minimized, and cancel out if they are graded properly. There are of course also ready made high voltage dividers, but they may lack the needed precision.
--- Quote from: EPAIII on July 12, 2022, 10:18:03 am ---A quick search did not yield any 300 Volt op amps. At least not such chips. Perhaps a discrete circuit? Or an op amp with a Voltage boost output circuit?
Any suggestions where I should look?
--- End quote ---
Apex Microtechnology and Burr-Brown were good sources for high voltage operational amplifiers. I have a PA42 on my desk which is good to a supply voltage of 350 volts but a high voltage operational amplifier is not a panacea because they are not usually designed with the kind of precision that you want, so trimming or offset correction would be required anyway.
A discrete voltage boosted operational amplifier would allow the use of a precision operational amplifier so no trimming would be required, but requires some design work for either the bootstrapping, level shifting, or high voltage gain stage.
If the high voltage output is over a very limited range, like only 300 volts, then a fixed level shifter can be used. Tektronix did this in their 50 and 100 volt regulators. The operational amplifier operated at +/-15 volts or whatever, and had a 50 or 100 volt zener diode on its output to "add" 50 or 100 volts to its low voltage output. This has the virtue of providing a gain of "1" inside of the loop despite the increase in voltage so frequency compensation is simplified.
Otherwise either the operational amplifier is bootstrapped so that its supply voltage follows the output, or a high voltage gain stage is added within the feedback loop. Either way works.
Note that all of the methods described above require care to avoid exceeding the common mode and differential input voltage range of the operational amplifier. This is particularly the case when a bootstrap configuration is used.
I would try the (poorly drawn) circuit shown below, extended to 300 volt operation of course. R6 and R7 are the precision divider.
mawyatt:
--- Quote from: EPAIII on July 12, 2022, 10:18:03 am ---A quick search did not yield any 300 Volt op amps. At least not such chips. Perhaps a discrete circuit? Or an op amp with a Voltage boost output circuit?
Any suggestions where I should look?
--- Quote from: tom66 on July 12, 2022, 10:00:16 am ---Why not just use precision resistors and an op-amp and power electronics circuit (or just a high-voltage opamp)? That way you only need one reference.
--- End quote ---
--- End quote ---
APEX PA441 or 443, 350V op-amps might fit your needs. We evaluated the PA443 awhile back, pretty good HV op-amp, however ended up selecting the TI OPA-462 instead for various other reasons.
Here's a evaluation HV AWG Buffer Amp with PA443 we developed awhile back.
Best,
EPAIII:
Thanks for that link. As I said, my search in the early morning hours was brief. I am sure there are others, perhaps not many, but others.
The latest posts have me seriously thinking about this approach. The REF chips are +/-0.05%. The 0.1% resistors suggested are above that level so that alone gives an accuracy hit. I would prefer 0.01% resistors or better, but then the price goes up. And I guess I can sort through a large batch of them with a meter of lesser accuracy because the goal would be matching values, not absolute ones.
Then there is the question of switching. If i keep the numerous Voltage steps that I presently envision, then a switching arrangement with around 30 positions would be needed. That is going to be a limitation if a single switch is to be employed. Multiple switches? I presently envision multiple, miniature, gold plated banana jacks, one for each output value. And yes, I know that's a bunch of banana jacks and $$$$. Perhaps something similar would work with a jumper cable. But, in usage, care would be needed so that a high Voltage is not produced while switching as that could damage a meter connected to the output which is set to a low Voltage scale.
And speaking of cost, the hi Voltage op amp in your link is expensive, but not so much so as 30+ REF chips. I need to see how it's cost compares with the needed resistors. And then, do I buy just one or do I get a spare? Will it still be available next year, in two years, in five years, etc. I can afford a few extra REF chips for maintenance spares in the event they are discontinued.
Back to switching those resistors. Some years ago I came up with a circuit for a decade resistance circuit that allowed it to be used as a decade pot. I believe I used twelve matched resistors for each decade and two wafers on a rotary switch which effectively jumped across two of those twelve with the next, lower decade. The problem was that each successive decade needed resistors that were 0.2X those of the previous one so only one decade could use 10xxx resistor values. Then 20xx and 40x ones would be needed (example 12 each: 10.00K, 0.01%; 2.0K, 0.1%; and then 400, 1%). But will that work with an op amp? I think yes, but it should be tested.
--- Quote from: David Hess on July 12, 2022, 07:57:08 pm ---
--- Quote from: tom66 on July 12, 2022, 10:00:16 am ---Why not just use precision resistors and an op-amp and power electronics circuit (or just a high-voltage opamp)? That way you only need one reference.
--- End quote ---
That is possible but presents calibration issues compared to stacked references.
Stacked references can be checked individually at low voltage. A precision high voltage amplifier depends on its high voltage feedback divider for an accurate output, and checking the accuracy of a high voltage divider is not easy. A low voltage test of the divider is not sufficient because at high voltages, the voltage coefficient of resistance becomes a factor and that can only be tested at high voltage.
The issues with a high voltage feedback divider can be overcome but require specific attention. What I might do is grade a bunch of the same resistor for precision, and then stack them to make the divider. Since all of the resistors will have the same voltage across them, the voltage coefficient of resistance will be minimized, and cancel out if they are graded properly. There are of course also ready made high voltage dividers, but they may lack the needed precision.
--- Quote from: EPAIII on July 12, 2022, 10:18:03 am ---A quick search did not yield any 300 Volt op amps. At least not such chips. Perhaps a discrete circuit? Or an op amp with a Voltage boost output circuit?
Any suggestions where I should look?
--- End quote ---
Apex Microtechnology and Burr-Brown were good sources for high voltage operational amplifiers. I have a PA42 on my desk which is good to a supply voltage of 350 volts but a high voltage operational amplifier is not a panacea because they are not usually designed with the kind of precision that you want, so trimming or offset correction would be required anyway.
A discrete voltage boosted operational amplifier would allow the use of a precision operational amplifier so no trimming would be required, but requires some design work for either the bootstrapping, level shifting, or high voltage gain stage.
If the high voltage output is over a very limited range, like only 300 volts, then a fixed level shifter can be used. Tektronix did this in their 50 and 100 volt regulators. The operational amplifier operated at +/-15 volts or whatever, and had a 50 or 100 volt zener diode on its output to "add" 50 or 100 volts to its low voltage output. This has the virtue of providing a gain of "1" inside of the loop despite the increase in voltage so frequency compensation is simplified.
Otherwise either the operational amplifier is bootstrapped so that its supply voltage follows the output, or a high voltage gain stage is added within the feedback loop. Either way works.
Note that all of the methods described above require care to avoid exceeding the common mode and differential input voltage range of the operational amplifier. This is particularly the case when a bootstrap configuration is used.
I would try the (poorly drawn) circuit shown below, extended to 300 volt operation of course. R6 and R7 are the precision divider.
--- End quote ---
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