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I'm trying to measure a variable resistance, RX, creating an AC voltage with two PWM and applying it to a wheatstone bridge. The PWM goes from 0 to 3.3v and to measure the voltage differential i'm connecting each arm to a InAmp to read after with a ADC. My problem is that my circuit is unipolar (+3.3v) so i'm looking for solutions to create an negative voltage source (-3.3v) to power the opamp in the negative rail to avoid saturation. I read about charge pumps, is there any other better solution ?

Thank you in Advance.

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  • \$\begingroup\$ @dim - it's an instrumentation amplifier not an op-amp. \$\endgroup\$
    – Andy aka
    Jul 21, 2016 at 11:58
  • \$\begingroup\$ The easiest approach is to use a DC-DC converter to give you +/-5V from +3.3, however a charge pump may be lower noise. Since you're stuck on AC measurement I presume you're wanting to measure with low or sub-microvolt resolution so that may be a concern. \$\endgroup\$ Jul 21, 2016 at 12:13

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Why you are looking for a "better solution" over already excellent solutions that industry already has invented? In situations like yours I use LM2776. It just mirrors your positive supply. The results are excellent: small PCB footprint, minimal number of passives, easy to get clean voltage.

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  • \$\begingroup\$ Thank you, I was afraid of using charge pumps because i thought it would produce a enough clean voltage for a rail-to-rail operation, but i will give it a try. Thank you. \$\endgroup\$ Aug 4, 2016 at 16:33
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Forget about the negative PWM excitation signal and bias the positive PWM signal up so that the AD620 is always dealing (on both inputs) with a voltage within its input common-mode range.

Note that the AD620 will need a supply of at least 4.6 volts so 3.3 V is completely out. So, if you went for a (say) 5 V supply, the input common mode range is between +1.9 volts and +3.8 volts. Read the DS for details.

Once everything is set up to work with the correct voltages, the AD620 output is: -

(gain)*(A-B) + ref_pin_offset

where A and B are the voltages at the inputs - note you don't need unipolar supplies or excitation to make this work but you do need adequate supply rails and attention to the common mode input voltage range.

You should also note that if Rx imbalances the bridge too much you will get a progressively bigger error term in your reading because, despite what you may have learnt, a quarter bridge (that's what they call them when only one resistor is active) has linearity errors: -

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Of course you could go for a constant current feed (still OK with PWM) you would halve that linearity error: -

enter image description here

It's also worth pointing out that slew rate limitations on dealing with fast edges (due to PWM) may well induce other errors from the AD620 so, have you considered the impact of this - maybe try simulating it and adding filters before the inputs to the AD620.

Remember that the AD620 is still pretty good with DC excitation having an input offset error of typically 30 uV.

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  • \$\begingroup\$ Hey Andy aka, thank you for your answer, you took my attention for several things i was not thinking on, but the crucial aspect of this resistor measurement is that is has to be measured with an AC signal excitation. Since in my circuit i only have a DC source of 3.3v and a MCU, i thought i could use this pwm approach to invert polarities and create an AC signal. If,as you suggested, I would bias this signal i would add DC component to it, right ? \$\endgroup\$ Jul 21, 2016 at 12:10
  • \$\begingroup\$ The DC bias would not be added to the output of the AD620 because it subtracts A input and B input and the same dc signal (at perfect balance) is on both inputs. \$\endgroup\$
    – Andy aka
    Jul 21, 2016 at 14:42
  • \$\begingroup\$ Hey Andy, thank you for your answer. The thing is that i cannot bias with on the input, its a requirement. I decided to use an AD623 and i'm supplying 3.3 and -3v3, although my input signals are in the common mode voltage range, the InAmp is not working properly, it only works properly when i raise supply voltage to -4.6 and 4.6, do you have any clue why ? \$\endgroup\$ Jul 27, 2016 at 14:01
  • \$\begingroup\$ Doesn't work properly means nothing to me. Be specific about the failings. \$\endgroup\$
    – Andy aka
    Jul 27, 2016 at 15:31
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You can set Vref on your AD620 to be mid-rail (you need to use a low impedance source like an op amp output or shunt voltage reference to do this), and not need the negative power supply. That said, the AD620 is not particularly well suited to this -- your inputs will still go too low. The AD623, though, should work just fine.

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  • \$\begingroup\$ Thank you for your answer. But since my ADC as V+ref of 3.3v , i would waste half of its measuring range, wouldn't I ? \$\endgroup\$ Aug 4, 2016 at 16:30
  • \$\begingroup\$ @user3689576 You can set Vref of the IA at anything that works for you, as well as the gain. Your issue will be the common mode input range on the AD620. \$\endgroup\$ Aug 4, 2016 at 16:35
  • \$\begingroup\$ Thank you. Yes, in the meanwhile since i posted this question i realized that too, that I could not do this measure due the common mode input range of AD620 or AD623. Anyway, this topic stopped to be useful for me, because it my variable resistance range changed dramatically, it turns out I have to be able to measure a much wider range , from 25ohms to 5k and Wheatstone bridge is definitely not an appropriate approach for it. \$\endgroup\$ Aug 4, 2016 at 16:55

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