Psoc 62 Opamp issue

[Anjali]

Hi Community,

I am using CY8C6247BZI-D54 microcontroller. I had used internal Opamp for amplification and I measured the output of the Opamp. For different input, I found different gain. What should it be?

Thank You.

[Vasanth]

Hi Anjali,

At what configuration are you using the Opamp ? When you say for a different input, are you telling you were trying to use a different GPIO ? Or did you feel that the gain change with change in input. Let us know the GPIOs used as inputs too.

Best Regards,

Vasanth

[Anjali]

Hi Vasanth,

Different input means different values (From 15mV to 35mV). Below is the measured values for Opamp.

The gain resistors are Rf = 50k and R1 = 1K for Noninverting Amplifier.

Input(mV)	Output(mV)	Gain
15	680	45.33333
16	726	45.375
17	778	45.76471
18	829	46.05556
19	880	46.31579
20	930	46.5
21	980	46.66667
22	1030	46.81818
23	1080	46.95652
24	1130	47.08333
25	1181	47.24
26	1231	47.34615
27	1282	47.48148
28	1333	47.60714
29	1383	47.68966
30	1434	47.8

[Len_CONSULTRON · Accepted solution]

Anjali,

I'm assuming that your Rf and R1 resistors are external to the PSoC6.

Your gain should be close to 51 for a noninverting Opamp.

You're getting about 47. This could be that your gain error is due to the accuracy of the resistors.

For example if your Rf resistor = 50K is -10% then the resistance then your actual resistance could be 45.0K    Then your actaul gain would be Av = 1+(45K/1K) = 46.

The reason you might be seeing the apparent different gains at different inputs is not due to gain error, but due to the +/-4mV input voltage offset (Vios)

If I take your results and adjust for different Vios values the closest match to give me close to the same gain across all readings is Vios = -1.5mV

Here is the adjusted table:

Input(mV)	Vios (mV)	Output(mV)	Gain
15	-1.5	680	50.37037
16	-1.5	726	50.06897
17	-1.5	778	50.19355
18	-1.5	829	50.24242
19	-1.5	880	50.28571
20	-1.5	930	50.27027
21	-1.5	980	50.25641
22	-1.5	1030	50.2439
23	-1.5	1080	50.23256
24	-1.5	1130	50.22222
25	-1.5	1181	50.25532
26	-1.5	1231	50.2449
27	-1.5	1282	50.27451
28	-1.5	1333	50.30189
29	-1.5	1383	50.29091
30	-1.5	1434	50.31579
		StdDev =>	0.064494

You will also notice that the calculated gain is much closer to the 51 predicted by your selected Rf and R1 values.

[Anjali]

Thank you so much.

[DennisS_46]

Len:
Easy.

Open switch (could be analog mux). Measure Vout_unitygain.
Close switch. Measure vout_with gain.

Voffset = (Voutwithgain - voutunitygain)/(1+r2/r1)

Make R1, r2 large compared to switch resistance, set gain to at least 16.

---- Dennis

[DennisS_46]

Thanks Len.
Another way to look at is to measure the differential gain from step to step. This is plotted below from excel.

Notice that the computed slope (gain) is very close to 50.36. If you expect 51 for the non-inverting gain,
then the measured vale is nicely within 1% resistor tolerances.

---- Dennis Seguine
PSoC Applications Engineer

[Len_CONSULTRON]

Dennis,

Thanks.  It looks like you used the "Trendline" function in the chart.

I used a 'brute-force' method to find the input offset of -1.5mV.   Knowing this offset, you can place the offset into the results to compensate for it.

Is there a best way to find the opamp's offset 'on-the-fly' in the FW?

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