OPA171-Q1: Fixed reference switches with respect to the Input frequency

SATHISH KUMAR KALIAPPAN

Prodigy 80 points

Part Number: OPA171-Q1
Other Parts Discussed in Thread: OPA171, OPA2990

Hello,

I am using OPA171 op-amp for simple comparator operation.

Condition:

Op-amp supply: +12V and -12V

Non-inverting terminal: 2.5V fixed reference (by voltage divider)

Inverting terminal: PWM input with 1KHz (0-5V)

Expected Output: Switched Output with +12V/-12V with 1Khz frequency (wrt to input PWM)

Issue: Non-inverting fixed reference is switching with respect to the input PWM frequency (0-2.5V). it seems to be abnormal to me. Experts please clarify if it is normal and why.

I have attached the circuit below. Please let me know if you need any additional information.

over 4 years ago

0 kai klaas69 over 4 years ago

Guru 140200 points

Hi Satish,

the OPA171 has protection diodes between its inputs. See figure 40 of datasheet.

Kai

0 Tim Claycomb over 4 years ago

TI__Genius 15410 points

Hi Sathish,

Kai is correct, the OPA171 has internal back to back diodes. When a differential voltage is applied to the device one of the diodes will turn on, clamping the differential voltage. Please see "Op Amps used as Comparators - is it okay?" for more information.

I recommend using the OPA2990 instead of the OPA171. The OPA2990 does NOT have internal back to back diodes and can be used as a comparator.

Thank you,

Tim Claycomb

Amplifiers

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OPA171-Q1: Fixed reference switches with respect to the Input frequency