Hello all,
I apologize in advance if I have not done a thorough enough job of searching the threads for my answer. I am in the process of building a system that can amplify by six orders of magnitude. I have done this by simply using two 10^3 stages of auto-zero offset op-amps (AD857x). This is a simplfication, as my first stage is actually 4 quad op amps which I then average and amplify simultaneously in the second stage using a single op amp.
I currently have a problem where all 16 of my first stage amplifiers are giving me different outputs, even though I have the same input on all of them (currently just a simple two-resistor voltage divider that provides me with a 300 microvolt signal for testing purposes). The circuit is currently set up for an overall gain of only 10^4, since I have to worry about thermocouple problems when I get down to the microvolt scale and would thus have trouble with initial testing. In any case, I am troubled since none of the amplifiers is outputting anything like 300 millivolts: they're all outputting more along the lines of 700 or 800 microvolts, although some are actually lower than the input signal for some reason!
I'm currently having trouble getting my circuit diagram to attach, so I feel that I should mention that the circuit ends with a two-pole RC lowpass with a time constant of 0.1 seconds. Also, the circuit is setup so that (in its final form) it will use Reed Relays to switch between 10^5 and 10^6 (10^6 is the default) using a logic signal. Also, the amplifier circuits are all inverting amps with negative feedback.
One last thing should be noted: when I make any of the first stage amps into a voltage follower, it works perfectly. As soon as I try to amplify by an order of magnitude, however, things go bad.
Sorry for the long post. I need this project to graduate, so any and all suggestions would be welcome. Thanks.
I apologize in advance if I have not done a thorough enough job of searching the threads for my answer. I am in the process of building a system that can amplify by six orders of magnitude. I have done this by simply using two 10^3 stages of auto-zero offset op-amps (AD857x). This is a simplfication, as my first stage is actually 4 quad op amps which I then average and amplify simultaneously in the second stage using a single op amp.
I currently have a problem where all 16 of my first stage amplifiers are giving me different outputs, even though I have the same input on all of them (currently just a simple two-resistor voltage divider that provides me with a 300 microvolt signal for testing purposes). The circuit is currently set up for an overall gain of only 10^4, since I have to worry about thermocouple problems when I get down to the microvolt scale and would thus have trouble with initial testing. In any case, I am troubled since none of the amplifiers is outputting anything like 300 millivolts: they're all outputting more along the lines of 700 or 800 microvolts, although some are actually lower than the input signal for some reason!
I'm currently having trouble getting my circuit diagram to attach, so I feel that I should mention that the circuit ends with a two-pole RC lowpass with a time constant of 0.1 seconds. Also, the circuit is setup so that (in its final form) it will use Reed Relays to switch between 10^5 and 10^6 (10^6 is the default) using a logic signal. Also, the amplifier circuits are all inverting amps with negative feedback.
One last thing should be noted: when I make any of the first stage amps into a voltage follower, it works perfectly. As soon as I try to amplify by an order of magnitude, however, things go bad.
Sorry for the long post. I need this project to graduate, so any and all suggestions would be welcome. Thanks.