Bipolar to unipolar converter for ADC input

Other Hardware General Electronics
1

Is it possible to design an OP-Amp circuit with the following requirements?
-- Single 3.3V supply voltage
-- Input voltage -10V to +10V
-- Output voltage 0 to 3.3V, or 3.3V to 0V. It doesn't care if the circuit is inverting or not.
-- Excluding the rails is acceptable, for example 0.3V to 3.0V
-- For 0V input voltage, the output voltage shall be in the middle at 1.65V.
So far, that's all easy. But now comes the requirement that makes things difficult:
-- For open input, the output voltage shall also be 1.65V.

2

What ?

3

What's unclear in the quoted text? An open input is an input that's not connected to any signal.

4

A signal source from -10V to +10V is always connected withe the inputs of op-amp. How can the signal lines be totally disconneted?

5

Yes it is.

6

Please share how you would do it.

7

Let's see your design and see if it will output 1.65V with a floating input.

8

Not possible due to stray charges and voltages induced by EMF.

The solution is very simple: "floating" inputs must be connected to 0V via a large resistor.

9

This circuit converts the +-10V signal to the 0.2V to 3.0V range. With 0V input the output is 1.6V, but with open input the output is 1.86V. I'm seeking a circuit that has the same output voltage for open or grounded input. Like a multimeter, which also shows zero for open or grounded input.

IMG_6919
IMG_69193597×2486 265 KB

10

I assumed your input was an AC signal and my design is AC coupled.
Is your input a DC voltage?

11

yes

12

Then it is a problem if all you have is a 3.3V supply

13

Unfortunately I must agree.

14

Have you measured those responses or analytically calculated?

You have not stated how much would be the output voltage if the input voltage is -10V?

15

both

input -10V output 0.2V
input +10V output 3.0V
The ADC input is 0 to 3.3V, but it's better to have a little distance from the supply rails.

16

Are they measured voltages or calculated? What type of op-amp have you used?

17

measured (approximately) and calculated. OPA2328 or similar. But that's irrelevant. The problem is that the output voltage is different for open and grounded input.

18

Are you measuring this to some equipment frame ground or are you measuring this from the other lead? Does your input even have a common ground type connection or is it a bi-polar voltage?

edit: I ask because most any RS-485 IC could do the translation for you.

19

Measured with respect to ground, which is the same as the negative supply of the OP-Amp. The OP-Amps output signal goes to an ADC input in a microcontroller.

20

Then you can use a single RS-485 chip to convert to a digital signal directly to the Arduino.