High internal resistance amplifier design issues

bigbat

High internal resistance amplifier design issues [Copy link]

 

The output of a biological electrode fluctuates in the range of about 300mV plus or minus 100mV, and the electrode output current is very small, about 50nA. There are four proposed design schemes:
1. Impedance matching first, then amplification. After the signal enters, the voltage follows, and then the same-phase amplification is performed. AD acquisition
2. Impedance matching first, then amplification. After the signal enters, the voltage follows, and then the anti-phase amplification is performed. AD acquisition

3. Amplify first, then impedance match. After the signal enters, it is amplified in the same phase,
and then the voltage follows. AD acquisition 4. Amplify first, then impedance match. After the signal enters, it is amplified in the opposite phase, and then the voltage follows. AD acquisition

The internal resistance of the signal is in the range of 6-7MΩ, and the current of 50nA can drive the op amp to work. Matching first and then amplifying may introduce serious noise when amplifying the current. If it is G ohm level, it is decisive to choose "matching first and then amplifying". So I am quite entangled.
Please discuss it with experts.

maychang

Of course, choose the first option.

In your last post, you used a single power supply. Inverting amplification requires outputting a negative voltage. How can a single power supply output a negative voltage? Negative voltage is also difficult to directly perform A/D conversion. So the second solution was rejected.

maychang

In fact, the "matching" and "amplification" mentioned by the OP can be done at the same time, that is, using one op amp.

bigbat

maychang posted on 2019-10-31 08:53 Of course, choose the first option. In your last post, you used a single power supply. Inverting amplification requires outputting a negative voltage. How can a single power supply output a negative voltage? Negative voltage is also difficult...

Thank you. Could you explain why solution 3, "Amplify first, then impedance match. After the signal enters, it is amplified in phase and then the voltage is followed. AD acquisition" is not recommended?

bigbat

maychang posted on 2019-10-31 08:57 In fact, the "matching" and "amplification" mentioned by the OP can be done at the same time, that is, using one op amp.

If the signal is in the uA range, there is no hesitation in designing an op amp. The key is that the signal is at the nA level, and I am afraid that the output of the amplifier will not reach the power to drive the ADC. I use the ADC that comes with the microcontroller, not the ADC with high impedance input, so I feel a little "guilty". It is safer to be safe. The input impedance of the microcontroller ADC is 50KΩ, and the current has certain fluctuations.

maychang

bigbat posted on 2019-10-31 09:16 Thank you, can you explain solution 3, "Amplify first, then impedance match. After the signal enters, it is amplified in phase and then followed by voltage. AD acquisition" ...

Regardless of whether it is in-phase or inverting amplification, since the amplifier circuit must have considerable voltage negative feedback, the output impedance of the operational amplifier is low enough (usually less than 1 ohm), so there is no need for "matching".

maychang

bigbat posted on 2019-10-31 09:28 If the signal is in the uA range, there is no hesitation in designing an op amp. The key is that the signal is at the nA level, and I am afraid that the output of the amplifier will not reach the power to drive the ADC...

Your signal amplitude reaches hundreds of mV, and the AD requires an input voltage that is generally no more than 5V, so the voltage gain of the amplifier is only a few dozen times. If the frequency is not very high, a single-stage op amp is enough. With a voltage gain of several dozen times, the output resistance of the op amp is usually less than 1 ohm, which can be completely ignored for the AD input impedance of tens of kilo-ohms.

bigbat

maychang published on 2019-10-31 10:27 Regardless of the in-phase amplification or the inverting amplification, since the amplification circuit must have a considerable voltage negative feedback, the output impedance of the op amp is low enough (usually less than 1 ...

There is some truth in what you said, but the op amps we use are not ideal op amps, so it is difficult to find high-current op amps at present, and the "internal resistance" of the circuit is still very high. What's more, the impedance of the ADC is nonlinear. In my practice, the ADC is not very stable without a follower. When tested with a battery, the fluctuation is very large. It is much better with a follower. This is purely the conclusion of my practice, for reference only! Statement: The microcontroller has its own ADC, not an external ADC. The external ADC chip has not been tested.

maychang

bigbat posted on 2019-10-31 10:43 What you said makes sense, but the op amps we use are not ideal op amps, so it is difficult to see high-current op amps at present. The "internal resistance & ...

When the op amp is working in linear amplification, it must have a very deep negative feedback, so the output impedance is very low, much lower than when the op amp has no negative feedback.

xiaxingxing

maychang posted on 2019-10-31 08:57 In fact, the "matching" and "amplification" mentioned by the OP can be done at the same time, that is, using one op amp.

The first one is "1. Impedance matching first, then amplification. After the signal enters, the voltage follows, and then the same-phase amplification is performed. AD acquisition". . . How is the "impedance matching first" matched? Isn't it just using an op amp with an input impedance much larger than the signal source impedance for amplification? Thank you!

maychang

xiaxingxing posted on 2020-3-17 20:38 The first type "1. Impedance matching first, then amplification. After the signal enters, the voltage follows, and then the same-phase amplification is performed. AD sampling...

This is not what I said, it is what the original poster said.

The input impedance of a voltage follower is roughly the same as that of an ordinary inverting amplifier, so you are right when you say, "Isn't it just a matter of using an op amp whose input impedance is much larger than the signal source impedance to amplify it?" This is exactly what I meant on the third floor.

bigbat

maychang posted on 2020-3-17 20:48 This is not what I said, it is what the OP said in the first post. The input impedance of the voltage follower is roughly the same as the input impedance of an ordinary in-phase amplifier, ...

Thank you. I have already made this circuit. I used a buffer first, that is, the impedance was equipped before the voltage amplifier adjustment. I then tried direct voltage amplification and it worked. I think: because the voltage amplification factor is not large, the input impedance is not very high beyond the amplifier itself. In fact, a small amplification factor also has a certain current amplification factor. It's just not large.

maychang

bigbat posted on 2020-3-18 08:01 Thank you, I have already made this circuit, using the buffer first, that is, the impedance is equipped in the voltage amplification adjustment. I then also tried the direct voltage...

Direct voltage amplification uses one less op amp than impedance configuration followed by voltage amplification, which can simplify the circuit and reduce costs.