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Laboratory Amplifier
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David Hess:
Using a x10 probe is definitely a problem for noise limited measurements; it immediately raises the input noise by 10 times and that is not even including the increase in Johnson noise from the increase in resistance.

On the other hand, a x10 probe isolates the cable capacitance which may be more important than noise and it increases the common mode and differential input range by 10 times.  The old Tektronix 7A13 differential amplifier even has a "x10 Vc" switch which adds x10 attenuation to the input and x10 gain to the amplifier so the sensitivity stays the same but the input common mode range increases by 10 times; this allows 10mV/div sensitivity over a +/-100 volt common mode range.

The above points to a serious limitation in most differential input amplifier designs; increasing the common mode range by attenuating the inputs commensurately increases the input noise.  Low input noise with high common mode input range is possible by simply using a design with high input common mode range which is what the Tektronix 7A13 does with an input stage which operates from +/-50 volt supplies for a base +/-10 volt input common mode range.  Typical oscilloscope FET input stages have a common mode input range of +/-250 millivolts which may indicate why the Rigol DS1000Z series has so many signal fidelity issues.

Operational amplifiers used as followers or a differential amplifier can do pretty well in this respect at low frequencies; +/-10 volts with +/-15 volt supplies is feasible and there are some parts which can do much better than this without an exotic circuit design like the LTC6090/LTC6091 although being a CMOS part, its 1/f noise is pretty poor and its AC common mode rejection is not any better than the instrumentation amplifiers we have discussed.  Also, differential input protection is still required despite the high common mode input voltage range.
MaxFrister:
Thanks for all the insightful comments.

I've completed rev. 3.  It has a number of minor changes including fixing the roll-off circuit and adding an ac input mode.   

Included is a preliminary pcb layout; it will likely change as I try to source the parts.
duak:
Nice design - should work well.

A few little things:

1.) if you're thinking to use X10 probes, I would add a balancing pot between the input resistors R101 & R102 of say, 1K with the wiper tied to ground.  This would compensate for any differences in the resistances of the probes and improve low frequency common mode rejection.

2.) I would isolate the power supply voltages for the first (INA) stage from the output stage with a series R and a bypass caps to common.  The output stage could draw enough current while driving a 50 ohm load to couple enough signal through the common power supplies to degrade the INA's performance. 
There are a number of paths:
 - the INA itself - look at the PSRR graphs,
 - the input protection diodes' reverse capacitance,
 - the offset control and opamp buffer,
 - the layout

My rule of thumb was a holdover from the bad old days of vacuum tube or discrete transistor amplifiers where one never powered more than two stages from one decoupling network.  Three stages almost always oscillated in weird ways eg., motorboating.  Opamps are much better but aren't perfect.  A colleague had something like three 40+ dB gain stages powered from the same point and it gave him grief until he realized what was up.

3.) Feature creep warning:  I'd implement a 3rd order filter as I found a sharper filter to be more useful.

Cheers,
MaxFrister:

--- Quote from: duak on December 11, 2018, 04:15:21 am ---...
1.) if you're thinking to use X10 probes, I would add a balancing pot between the input resistors R101 & R102 of say, 1K with the wiper tied to ground.  This would compensate for any differences in the resistances of the probes and improve low frequency common mode rejection.

--- End quote ---

Good idea, but see answer to #3.


--- Quote ---2.) I would isolate the power supply voltages for the first (INA) stage from the output stage with a series R and a bypass caps to common.  The output stage could draw enough current while driving a 50 ohm load to couple enough signal through the common power supplies to degrade the INA's performance. 

--- End quote ---

Good idea.  I think I can accomplish it without major damage to the existing layout.  Where would you recommend I set the -3dB point on the filter?  Would you include the offset control for the INA on the filtered supply?


--- Quote ---3.) Feature creep warning:  I'd implement a 3rd order filter as I found a sharper filter to be more useful.

--- End quote ---

For my purposes, I think first-order filter is okay.  Notice that that third gain stage is already complicated: Summing junction for dc offset, gain of 6dB to 50-ohm impedance output,  low-pass filter, and output buffer.


Feature creep?  I'm good at that.  I've learned from experience that at some point I have to say "good enough" and move on to the next stage -- ordering the pcb in this case.  Otherwise, the project keeps expanding in scope until (a) I can not longer accomplish it, or (b) I lose interest.  But here is my wish list for version 2


* Relay switches to minimize noise pickup from wires running to the front panel
* MCU for external computer control
* Digital pots to eliminate the expensive 10-turn pots and MCU control
* Meter on output for setting dc offset (the original had this).
* Clipping indicator light
* Uncalibrated gain indicator light
* Reverse phase switch
and of course lower noise design.
duak:
Max, regarding the power supply filtering for the INA.  You're using 10u caps already.  If the series R is 22R, the corner frequency will be 800 Hz and will give a minimal DC drop.

I would definitely power U102 and perhaps U103 and U104 from the filtered INA supplies.

I just noticed that U204, the -5V buffer, has a 10uF bypass cap on its output.  Most opamps with large capacitive loads are unstable without a series resistor.  U205, the +5V reference is probably stable with a capacitive load but should be checked.

Also, RV101 will be exceedingly touchy on the higher gain settings because of the high gain of the following stages.  Just thinking out loud here, what purpose does it serve?  I can sort of see a need for canceling out an offset before being further amplified.

Cheers,
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