AOP Instrumentation

Question

Hello, 
 I would like to find an instrumentation AOP with a gain of around 1500. This AOP accepts low amplitude signals at the input around 1&micro;V to 320&micro;V. Do you know if there are comparator links to find this kind of component ? I use this component in the audio frequency band (10Hz-20kHz). 
 With a weak PSRR, LOw noise and the supply voltage is +3.3V and -3.3V. 
 Thank for your helps 
 Regards.

Hooman · Accepted Answer

Hi 206_RC , 
 Yes, the input noise voltage AND the "0.1Hz to 10Hz" Vnoise (Vpp) are good parameters to choose for when trying to find a part that can extend the lower detection voltage range to 1uV, as in your case. 
 For example, you can use this list sorted for lowest noise: 
 
 Excel file showing these parts: 
 XLSX 
 I took the lowest noise device in that list (AD8428) and simulated its output noise as shown below. Gain is 2,000V/V. I've filtered the output to 20kHz -3dB Bandwidth using C1: 
 
 In your case, the output noise is a combination of 1/f noise (0.1Hz to 10Hz region) as well as the broadband / flatband noise voltage of 1.3nV/RtHz and simulation shows a RMS value of 591uV_RMS. With 1uV hydrophone input signal and a gain of 2,000V/V, the SNR will be: 
 SNR = 20* log (1uV_RMS * 2,000V/V / 591uV_RMS) = 10.6dB 
 So, you should be able to decipher that low level of input voltage and stay 10dB above the noise floor. 
 Here is the simulation file you can use (Extract All before using): 
 AD8428 Hydrophone amp with Filter EZ 7_12_22.zip 
 
 Regards, 
 Hooman)

Hooman · Answer

Hi 206_RC , 
 
 206_RC said: Your screen shot about the AOP choice is too small and I can't see the parameters you entered : 
 Response: Yes, sorry about the low resolution. You can use the Excel file I also attached earlier for the same exact information. 
 206_RC said: This term '1/f noise (0.1Hz to 10Hz region)' what you mentioned I don't quite understand, 
 Response : 1/f noise is the noise in the lower frequencies region. It's called 1/f noise as it typically increases when frequency is reduced. In your case, as you're going up to 20kHz, the 1/f noise region gets to be negligible compared to the flatband / broadband noise of 1.5nV/RtHz (sorry, my parametric table shows 1.3nV/RtHz but you found that the datasheet shows 1.5nV/RtHz which is the number to use). 
 206_RC said: What does E1 mean and the value -1 
 Response : I used E1 to generate a diff input for the In Amp. I wanted to create a voltage which is 180 deg. out of phase, so I used "-1". 
 206_RC said: And your power supply is not within the range recommended by the manufacturer 
 Response : Yes, you're correct. You need to select a device from the Excel file which can accept the +/-3V you're using. 
 206_RC said: No, in fact in my Hydrophone there is an integrated preamplifier so I think the output impedance of this preamp is very low. 
 Response : Then selecting for the lowest Voltage Noise is the right approach. 
 Regards, 
 Hooman

Hooman · Answer

Hi 206_RC , 
 206_RC said: In fact, probably this component may suit me, I'll have +5V and -5V also in my board. But whe did you use this assembly with 2 generators V3 and E1 ? 
 Response: The two generators used in simulation is just to apply a fully differential, ground-referenced input. In your application, your hydrophone plays that role. 
 206_RC said: Why can't you just use one sine wave generator to do the same thing ? Like this : 
 
 Response : What you've shown is different in that V1 has no CM reference. Some simulations have issues with sources which don't have a ground reference. That's why I used "E" to generate an equal and opposite signal at the input which does have a CM reference to ground. 
 206_RC said: Another point, I would have liked to find this same component (same technical characteristics) but with a variable gain up to around 5000 (because I don't know exactly the desired gain). 
 Response : Other devices except AD8428 in the Excel file I shared with you earlier allow a range of gains, as shown here: 
 
 So, all you need to do is to select those devices which show min / max gain range. 
 
 Regards, 
 Hooman

Hooman · Answer

Hi 206_RC , 
 
 206_RC said: With a Ibias 50n, I have also a offset ? Without R2 I have -50mV (It's very important). what is the simulation assembly that allows not to have an offset and a gain of 2000? Since you advised me this AOP to have no offset (or very low) ? you advised me this AOP according to my constraints it seems to me not ? 
 Response : AD8428 has up to 25uV input referred offset voltage (Vos) for the B-Grade (A-Grade Vos is higher at 100uV). The LTspice model has the 25uV built-in. I've cancelled this by adding V_offset to show this below (for demonstration only): 
 
 Here is the simulation file: 
 AD8428 Hydrophone amp with Filter EZ 7_20_22.asc 
 In your application, you could add a variable offset voltage (+/- 250mV) to the REF pin to compensate for this Vos offset voltage. 
 
 Simulation file: 
 AD8428 Hydrophone amp with Filter offset on REF pin EZ 7_20_22.asc 
 Regards, 
 Hooman