Input signal is affected after connecting to the differential amplifier. How do I fix that?

Question

I am currently using a differential amplifier, but the moment I connect the input to the differential amplifier, the input voltage decreases. Below is the diagram.

Update:

The input comes from a RMS IC module that has an output impedance of 12.5kohm. The RMS IC module outputs max voltage of 900mV. Per everyone recommendation, I searched for instrumentation amplifier but unfortunately, I cannot seem to find an instrumentation amplifier as it looks like it has a minimum voltage requirement (looking at the diamond plot.)

• I am using a single 9V power supply
• The input voltage varies from 100mV to 900mV.

Based on this, I cannot seem to find an instrumentation amplifier but it looks like it work well with a dual supply. Can anyone recommend me an instrumentation amplifier that works well with an input voltage range of 100mV to 900mV with a single 9V power supply. I hope to hear your response.

I have thought of using a voltage follower but I would like to keep it down to a single op-amp if possible. I hope to hear your opinion.

Answer 1

The differential input impedance of your amplifier is 20k. Since the output impedance of your signal source is 12.5k, you will lose a considerable amount of signal due to the voltage divider effect: the signal will be reduced by a factor of 20/(20 + 12.5) which is 0.615. One solution is to increase the size of your resistors to 100k or more. However, depending on the op amp you are using, this could lead to problems due to input bias current and current noise. A better solution, that uses only one device, is to replace the op amp with an instrumentation amplifier which can provide a much higher input impedance without bias and current noise problems. Analog Devices, among others, makes a variety of such devices.

Answer 2

You can make your circuit using a single IC, but not single op amp. In the picture you can see the instrumentation amplifier. Since the + inputs have extremely high impedance, they won't affect your source.

But there are possible problems if the common mode voltage is out of range VS supply voltage & gain, or even single supply operation. Not all instrumentation amplifiers (IA) are suitable for your demand, but unfortunately you didn't mention the nature of the signal voltage, common mode, frequency,....

The diamond plot

Please read the main limitations of the IA. You can use the Analog Devices online tool to estimate the IA from their portfolio, just as an example.

Perhaps your application (unknown) would benefit of the use of indirect current feedback amplifier.

Answer 3

^{simulate this circuit – Schematic created using CircuitLab}

Here is a circuit with high input impedances (so as not to affect the input voltages) and using only a single quad op-amp package.

You can use a better performance op-amp than the LM324, depending on what your requirements are. Note that the 120mV source is pulling the OA3 op-amp output up with a current of about 25uA. The LM324 has a current sink of 50uA (nominal) to pull the output down close to the negative rail, so it might be okay, but I've added R5 as a bit of insurance. Similarly the voltage at the inverting terminal of OA3 is 900mV and there is therefore 30uA flowing through R4 (even worse), hence R6. Even with no current sink the lower voltage limit is then V2 = 82mV for V1 = 900mV.

If you consider some other kind of R-R output single-supply or RRIO op-amp you should look at the output swing under load to guarantee it works over the range of V1 and V2 you have in mind. Getting the outputs to reliably and stably swing very close to the supply rail needs a bit of care.

Answer 4

You may use a single supply quad opamp like LM324 or a modern type like the rail to rail op amp LM6144 as suggested by John Doty.

https://www.onsemi.com/pdf/datasheet/lm324-d.pdf

Look to Figure 13. High Impedance Differential Amplifier on page 8.

In the equation eo = C (1 + a + b) (e2 - e1) set a = 0, b= 0, and C = 1 to get eo = e2 - e1.

Note the ouput voltage low limit of maximal 20 mv.

Of course you should read the full datasheet very carefully!