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Laboratory Amplifier
MaxFrister:
Laboratory Amplifier
I’m working on a general purpose lab amplifier. It started with the designs from The Art of Electronics (TAOE) vol 2 and 3 but I’ve made a few changes in requirements.
I have a few questions and would also be interested in any criticisms upon the current design.
Design goals include:
* Input impedance of 10M-ohms
* Fully differential (isolated) input
* DC offset to +/-10V from a single control
* Output in-phase with input
the original book designs provided:
* Calibrated and variable amplification
* Bandwidth from DC to 100kHz
* Bandwidth limiting
* Fault tolerant to input of +/-150V
* Output impedance of 50-ohms
Questions:
* Is 10M input impedance achievable with the AD620 or will the offset currents result in an uncorrectable offset?
* The AD620 instrument amp requires a DC ground reference for “truely differential” inputs to keep them from floating to the rails. R2 and R3 attempt to provide it but adversely effect the 10M input impedance. Is there a better way?
* C15 and C16 are included in TAOE vol 2 but not 3 (which also feature different front end opamps, LF411 and OPA627). Why are they there in 2 and gone in 3?
* TAOE has the offset adjustment for the LF411 referenced to V+ but the National datasheet says to reference it to V- (source or sink current into the long-tail pair). Does it make any difference?
* R13 and C24 are not in TAOE Vol 2 but are in Vol 3 (again the op amps have changed from LF411 to OPA627). The text say they are there for stability but I’m not clear on how those values were arrived at? Are they necessary for the LF411 as well?
By the way, I’m planning on using LF411 ($0.00 each) instead of OPA627 ($25.00 each).
Conrad Hoffman:
Haven't seen the original circuit. Don't know why they made the changes between revisions, but I'd expect the performance of a "jelly-bean" 411 and the fairly exotic 627 to be quite different. The input currents are a factor of 10 different, 5 pA for the 627 and 50 pA for the 411. Then you have the 620 instrumentation amp at 1 nA. What did they use originally? All opamps need to be referenced to ground on the inputs for differential operation. You might have to select opamps to work with 10M, or use the better opamp. Look at page 17 of the 620 data sheet where they use 100 kohm to reference the inputs. I think this amp is best suited for lower impedance sources. BTW, most lab amps would use two BNCs, with the shells at ground and matched paths to the opamp inputs. If you want the high frequency common mode rejection to be any good, the signal paths, including probes, have to match. Also, paint or keep the diode protection network in the dark.
awallin:
an open hardware modernized version of the SRS 560/570 would be a nice project!
https://www.thinksrs.com/products/sr560.html
at least some years ago the schematic was available from SRS.
jh15:
My lab space is quite crowded, would like to have a lab amplifier :)
MaxFrister:
The original text-book example is similar to what I've given but omits the differential input.
I've made a number of changes to the first revision.
* Replaced the in-amp with an ad8220. This has impressive specs, reasonable cost, but is smaller than I usually like.
* Added 2 input bnc jacks referenced to ground. I left the original single isolated BNC since I often just need to convert from high impedance to 50-ohm without amplification.
* Changed the protection diodes with low leakage JFETs. I didn't know about the light-sensitivity of 1n4148.
* Fixed the +/-10V part of the circuit.
I've been looking for the schematic for the SR560. A fragment is available in TAOE. It appears to set the differential input impedance to 200Meg!
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