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I am making a differential amp with a non-inverting amp with a gain of 4.9 with the use of an AD822 which is a dual rail-to-rail op amp.

My input signal on the diff amp is on the range of 2.5V to 3.45V and then i am subtracting 2.47V from it. The output then goes to the non-inverting amp for it to be scaled up to about 0.15V to 4.8V for the arduino ADC to read it. My problem is I'm not having the gain of 4.9 which I want, im getting a gain of 6 and above at my output.

I am thinking that buffering the diff amp output before going to the non-inverting could solve the problem but that would need another rail-to-rail op amp. What other solutions could I do?

FYI, the input from the diff amp is a load cell output using an in-amp(AD620). And the 2.47V is provided by a shunt regulator(TLV431) enter image description here

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  • \$\begingroup\$ Are you sure it's a gain of 6 and not just a constant dc offset that makes you think it's gain is 6 - I would also recommend LTSpice (free). \$\endgroup\$ – Andy aka Jan 12 '15 at 9:18
  • \$\begingroup\$ it might be but im not sure. Im planning to apply the gain on the diff amp and then use the other half as a voltage follower with a sallen key low pass filter as the output will be going on the ADC \$\endgroup\$ – Rhonald Rei Pahayac Jan 12 '15 at 9:26
  • \$\begingroup\$ Applying ideal voltage sources at both inputs the circuit works with a gain of 4.9 (simulation with AD820). Are both input signals low-resistive voltage sources? \$\endgroup\$ – LvW Jan 12 '15 at 10:42
  • \$\begingroup\$ it looks ok to me. the only thing that bothers me is the supply, 5V is right on the edge of the allowable range. is your powersupply good enough? they don't guarantee that won't turn to custard with a 4.99V supply. \$\endgroup\$ – Jasen Jan 12 '15 at 11:04
  • \$\begingroup\$ @LvW the input at the non-inverting pin of the differential amp comes from the output of an in-amp buffered by an op-amp. Also, the 2.47V at the inverting pin comes from the output of a shunt regulator buffered by an op-amp. The difference between the two voltages seems to be always right, but after it is scaled up using the non-inverting, some offset is introduced and added up and is not matching an expected output with a gain of 4.9 \$\endgroup\$ – Rhonald Rei Pahayac Jan 12 '15 at 13:03
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Since this is a jfet input op-amp be aware that input voltages slightly over the + rail supply can quickly damage the part. Your circuit seems protected as is, but if you did any substantial poking/probing you might want to verify that the part in the circuit is still ok.

While this op-amp is listed as being a rail to rail part it doesn't absolutely reach the rails. Per the spec the low end will only go to within 5mv of the - rail and 10mv from the + rail. (See the spec sheet section "Output Characteristics", page 18.) Other odd things happen when the output is very close to either power rail.

A potential source of larger errors may be due to the input error voltage when the output is within 300mv of either power rail. (See spec sheet figure 13, page 12). While the error is normally in the uV range your minimum output of about 30mv would go well off the chart on the high end. With a 10k load you would need to keep the output at about 120mv above the - rail to minimize the error, (I'm extrapolating the chart between RL=20k to 2k). This chart uses an example with +5v-5v supply rails, using only +5v-0v might be even worse.

Also be sure you don't have any significant AC noise on your inputs. If you were expecting all DC outputs maybe you debugged with a DVM on DC. Use a scope to check for AC noise. Just a few mV of noise would be very significant at your lowest input levels. If there is any significant AC coming in you could put caps across the 10k feedback and the 10k going to GND, (of the diff amp). The lower the noise frequency the larger cap values would need to be used to filter it.

You may want to decrease the 2.47v reference a small bit to keep the lowest output voltage farther away from the - rail (0v). Since you say your 2.47v reference is buffered by another op-amp you could put a multi-turn pot ahead of that input to give you an accurate way to calibrate the output voltage range.

Too large a cap on the final output (going to the A/D input) might also cause problems for this op-amp.

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