I have a circuit which has an input of voltage of the order of micro. I want to convert it into volt order. I am using an instrumentation amplifier AD620 with gain of 1000 to first convert it into milli order. And then I am using TL084cn as a comparator whose one input is the milli order voltage and the other is grounded. The +Vcc and -Vcc is 10 V. Also the frequency of operation is between 8-40 Hz. The question is that theoretically this is possible but before I implement it practically I would want to know whether I could face some issue for such sudden abrupt change in the order of the voltage. Also, keep in mind please that I don't actually care about the shape of the signal cause in my project the information is in frequency and not in amplitude.
closed as unclear what you're asking by Chris Stratton, Elliot Alderson, Sparky256, Lior Bilia, Voltage Spike Feb 8 at 17:44
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If you use comparator then you may not get one to one mapping of input voltage and output voltage. If your input is always +Ve voltage then your output of comparator will always be stuck at +Vcc as other end of your comparator is grounded. You may need low noise high gain amplifier to do so. Also note that high gain will amplify noise in the signal too.
Common opamps have Rnoise of 1kohm, perhaps 10kohm; these opamps will in 100 cycle-per-second (100 Hertz) bandwidth produce 4nV * sqrt(100) or 12nV * sqrt(100) equivalent input noise, or 40 nanovolt for the 1Kohm Rnoise opamp and 120 nanovolt for the 10kohm Rnoise opamp.
If you amplify these noise levels by 5,000,000X to produce logic levels, you will have 200 mVRMS or about (6.2X more, for 1ppm noise peaks) 1.2 volts riding on the 5 volt logic output, assuming no hysteresis in the comparator --- this for the 1kohm Rnoise opmap.
For the 10kohm Rnoise opamp, the output noise voltages will be 3.16 or sqrt(10) larger, and the 1ppm noise peaks will be 1.2 volts * 3.1 == 3.7 volts, for a 5 volt logic output.
Your goal is achievable, but spend the extra power (in that first amplifier) and use an amplifier with 1 or 2 nV/rtHz (60 to 240 ohms Rnoise) noise density.
I used 100Hz to ensure your 40Hz requirement can be achieved.
In addition to the other excellent observations, input offset voltage of the AD620 is 50 microvolts max. This is the order of your signal, if not bigger. After you amplify it by 1000, you may have a 50 millivolt offset, in either direction. You will need to deal with this.