Please give me some advice, is this an amplifier impedance issue?

bigbat

Please give me some advice, is this an amplifier impedance issue? [Copy link]

Currently, there is a biological electrode for measuring toxicity, and the electrode measurement characteristics are correlated with voltage. The electrode output fluctuates in the range of about 300mV plus or minus 100mV. However, the electrode transmission current is very small, about 50nA. According to the electrode parameters, it is planned to use a "common-phase amplifier circuit" of a CMOS precision rail-to-rail op amp, and then the voltage follows. The amplification factor is 6-8. The current difficulty is that the range level of the feedback resistor cannot be determined?
Please give guidance from experienced experts. Should the feedback resistor be in the MΩ level or the KΩ level? Or something else? I want to use the MΩ level, but I am afraid that I can't push it. KΩ feels that the current at both ends is unbalanced and the amplifier is unstable?

maychang

Let's not talk about the feedback resistor for now. Calculate how much voltage the 50nA current output by the sensor can generate on your R3.

50nA*7kohm=0.35uV. That is 0.35 microvolts! That is one millionth of 300mV!

bigbat

maychang posted on 2019-10-29 12:09 Let's not talk about the feedback resistor first. Calculate how much voltage the 50nA current output by the sensor can generate on your R3. 50nA*7kohm=0.35uV. It is 0 ...

What if the resistance of R3 is infinite? Or it is not necessary at all.

maychang

bigbat posted on 2019-10-29 12:45 What if the resistance of R3 is infinite? Or simply don't have it.

The resistance of R3 is infinite, which means the same as "don't".

Even if R3 is removed (open circuit), the op amp input current still needs to be considered. The op amp input current (bias current) must be much less than the 50nA output by the sensor.

bigbat

maychang published on 2019-10-29 13:02 The R3 resistance is infinite, which is the same as "don't". If R3 is removed (open circuit), the input current of the op amp still needs to be considered. The op amp's input...

When choosing a CMOS op amp, we have to consider the input impedance. The input impedance of a BJT op amp is about 40M, so it cannot be used. However, the current reaches the level of 2.5pA. The current of CMOS can reach a thousand or even ten thousand times that of BJT, so it should be possible. Let's go back to the original question. Is this feedback resistor at the K or M level?

maychang

bigbat posted on 2019-10-29 13:47 When choosing a CMOS op amp, we need to consider the input impedance. The input impedance of a BJT op amp is about 40M, so it cannot be used. However, the current reaches 2.5pA...

"Let's go back to the original question. Is this feedback resistor K or M level?"

For the common-mode amplifier, the feedback resistor has nothing to do with the input impedance, and k ohm level is usually selected. M ohm level resistors have high noise and are not as stable as k ohm level resistors.

maychang

bigbat posted on 2019-10-29 13:47 When choosing a CMOS op amp, we need to consider the input impedance. The input impedance of a BJT op amp is about 40M, so it cannot be used. However, the current reaches 2.5pA...

The op amp used in the first post is AD8073. This op amp is a current input op amp with very low input resistance, which is not suitable for your requirements.

maychang

bigbat posted on 2019-10-29 13:47 When choosing a CMOS op amp, we need to consider the input impedance. The input impedance of a BJT op amp is about 40M, so it cannot be used. However, the current reaches 2.5pA...

In addition, the first circuit uses a single power supply and the input signal is 200mV to 400mV. When selecting an op amp, the input common-mode voltage range of the op amp must be considered.

bigbat

maychang posted on 2019-10-29 16:00 The op amp used in the first post is AD8073. This op amp is a current input op amp with very low input resistance, which is not suitable for your requirements.

I randomly chose AD8073 for drawing, and the op amp I was going to use was LMC6482AIMX/NOPB. But I don't know what NOPB means? I looked at its parameters

Rail-to-Rail Input Common-Mode Voltage
Rail-to-Rail Output Swing (within 20mV of 100kΩ load)
Guaranteed 3V, 5V and 15V Performance pH Meter, Current Source
Excellent CMRR and PSRR: 82dB
Ultra Low Input Current: 20fA
High Voltage Gain (RL = 500kΩ): 130dB
Specified for 2kΩ and 600Ω loads

Minimum input current: 20fA, which I think is much higher than my 50nA, so it should be OK.

Thank you, boss, for being so patient!

Thanks!

maychang

bigbat posted on 2019-10-30 08:58 AD8073 is a random choice for drawing, and the op amp device I plan to use is LMC6482AIMX/NOPB. But what does NOPB mean, I don't know? ...

The bias current parameter of this op amp far exceeds your requirement. As long as the input current is as low as pA, the measurement accuracy requirement is met.

As for what NOPB means, you need to read the instructions provided by the manufacturer to find out.