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I'm designing a voltage measurement adaptor. And my signals are PWM type which ranges from 0-36V. I thought of using Op-amp to step down to 0-5V range and use an ADC to convert analog signals into digital.

To have a gain of <1,i'm using an inverting Op-amp (LT1630) where I can have split supply of (+18V to -18V) but then I have problem of connecting my input signal to a common ground zero(to avoid floating of input signal), so is it possible to give -36V at negative reference and ground the positive reference ?

LTspice simulation gives expected output (Gain of less than 1) with -18V at negative reference and Grounded positive reference for 36V input. How is that possible? enter image description here

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  • \$\begingroup\$ You can just use a voltage divider (2 resistors). What is the input impedance of your ADC - don't you want to get the average value? if so, add a capacitor. If not, you only need a digital input not an ADC. \$\endgroup\$ – Icy Nov 2 '15 at 12:07
  • \$\begingroup\$ I'm very confused on what you are doing with all those voltages, I'd just use a simple voltage divider and if needed an opamp buffer behind it. If you want to get a PWM signal input, you can just use a digital input, probably one of a timer-counter module to decode it. \$\endgroup\$ – Arsenal Nov 2 '15 at 12:10
  • \$\begingroup\$ @Arsenal : In my measurement environment, I’m not supposed to use voltage divider, because DUT i use in my test environment will misunderstand divider resistor of measurement adaptor as load. I chose opamp because of its very high input resistance. \$\endgroup\$ – zuccy1 Nov 2 '15 at 12:20
  • \$\begingroup\$ @JayaramVignesh your very high input resistance is currently at 34kOhm. You'll need a buffering opamp first, but if you have it buffered, you can then use a resistor divider. How small is the current to be detected as a load? \$\endgroup\$ – Arsenal Nov 2 '15 at 12:29
  • \$\begingroup\$ @Arsenal Thanks for your idea of using buffer. Current should not be more than few micro amps. In my circuit, i would connect an ADC next to Op-Amp which has got very high input resistance, so that current flow in the circuit is very much restricted. My doubt is that, do supply rails of Op-Amp can decide only the range of input voltage or the range along with sign(+-). Because in my above circuit i have given supply only to -ve reference but the voltage input between 0-36V and it worked.How? \$\endgroup\$ – zuccy1 Nov 2 '15 at 12:55
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Inverting Op-Amp with gain < 1 will always tend to give output only in negative polarity and for that it draw power from negative supply of Op-Amp.If we use Single supply Opamp negative supply line of Op-Amp is grounded and output will be zero or low positive voltage.

We can attenaute positive voltages using inverting Op-Amp with dual supply as given in the picture using extra reference voltage Vref.enter image description here

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