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| Debugging differential charge amplifier |
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| SethGI:
Hi everyone, I realize this is a long post, but I wanted to make sure I'm giving all the details possible so people can help me. I'm working on a DAQ for a piezoelectric hydrophone (Teledyne TC4013, 3nF ESC element). Because it's a piezoelectric sensor, a charge amplifier makes the most since. Because I'm using it to drive an ADC (Analog Devices ADAR7251) it needs to be fully differential. Further, I only have a single 3V3 supply. So, in short, the amplifier needs to be a charge amplifier which does single-ended to fully differential conversion, on a single supply. I passed this bit of the design off to a (very competent) peer, but now I've assembled the board and am struggling to get it to work. I just can't produce an output signal with the hydrophone as an input. I list my debugging steps below. I'm hoping for some debugging tips as we're both stumped by this. The amplifier is designed in two stages: a charge pre-amplifier, and then a normal voltage opamp to do a bunch more amplification. Both stages use a TI THS4551 opamp. I've attached schematics from both the preamp and 2nd stage, although I'm quite confident the problem is in the preamp (then again, if I was never wrong I wouldn't be posting here in the first place). Here are all the measurements I could take: -Vdc=125mV at the positive hydrophone terminal (measured at R_2). Here, using the hydrophone and pinger I can measure a signal which is ~20mVpp at 20KHz. I couldn't detect that signal anywhere else in the system, so all I can report further downstream is the DC voltages at various locations. -V_cm = 1.9V For the preamp: -V_out+ ~= 10mV -V_out- ~= 3.3V For the 2nd stage: -V_out+ ~= 3.2V -V_out- ~= 20mV Not being an expert, those voltages seem strange. With no detectable signal passing through, shouldn't all the output voltages be at Vcm? Then again, maybe that little voltage at the input to the amplifier is just getting blown up and saturating the opamp (is this the problem????). So far, these have been my debugging steps: -Using a (voltage) function generator to pass a known signal through the system. It generally works, with some small clipping errors that are (I believe) irrelevant to the current problem I'm having. If that persists and I can't fix it, it'll be it's on post. -I removed the capacitor from the feedback loop in the preamplifier, so it's just a bog-standard opamp circuit. Still no output detectable. -I decreased the capacitance of the capacitor in the feedback loop of the preamp to 18pF (the smallest which I had on hand), and still no output detectable. Please let me know if there's anything else I can provide! I've really been banging my head against a wall on this one and am looking for any guidance at all as to what could be going wrong. Thanks in advance! -Seth |
| duak:
Seth, to confirm, you have 125 mV DC on the input connector J1? Isn't the differential DC gain about 3000X? It looks like the 1st stage amp is driven hard into saturation. What are the output voltages when J1 is shorted out? Perhaps temporarily reduce R1 & R4 to 10K to get a handle on things. |
| magic:
Just a side remark, why bother with all that differential stuff if you could build a single-ended amp and convert to differential at the very end? I'm not familiar with the stuff you use but piezo sounds like something very high impedance and I presume your diffamp chip has bipolar inputs. Where is the bias current supposed to go? |
| SethGI:
--- Quote from: duak on April 05, 2020, 04:20:56 am ---Seth, to confirm, you have 125 mV DC on the input connector J1? Isn't the differential DC gain about 3000X? It looks like the 1st stage amp is driven hard into saturation. What are the output voltages when J1 is shorted out? Perhaps temporarily reduce R1 & R4 to 10K to get a handle on things. --- End quote --- Thanks! I'll try that to when I can get back to the lab tomorrow. Eventually, that degree of amplification will be necessary, but good call on reducing it for now. |
| duak:
When I skimmed over the data sheet last night I missed seeing the input bias current: 0.55 to 1.5 uA. This will cause a significant voltage offset across the input because the '+' input current goes right to ground whereas the '-' input current must go through R1, a 1M0 resistor. I think you'll have to reconsider the choice of devices and topology and use a MOS input device here. |
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