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OPA2180: OPAMP has unexpected behaviour for step input

Part Number: OPA2180

Hi Team,

Good day, we received query from customer and seeking support regarding OPA2180. For me to not miss any information I'll copy the query below.

We are using OPA2180IDR to buffer a signal representing current. The signal is referenced to gnd "0VA", and is negative. The amplifier is configured as an inverter with gain of 1, with VCC=+12V, VEE=0VA. The input signal is waveform is a square wave, 5kHz, stepped from -30mV to -4.0V to -30mV. The output is expected to be +30mV to +4.0V to +30mV. However, we do get this expected waveform.

To assist you to understand the problem better, I have attached images that show schematics, and scope waveforms. Brief description:
1. System ground (earth) is node marked "0VA".
2. Signal generator GND is connected to "0VA".
3. Scope GND is connected to "0VA".
4. Scope CH1 is connected to node "SHUNT-NEG", refer attached ckt 1 of 3.
5. Scope CH2 is connected to op-amp output, node "X42", refer attached ckt 2 of 3.
6. Bypass cap (1uF/25V) is connected very close to package pins, ref attached ckt 3 of 3.
7, Signal generator set up is shown in attached image. Note that sig gen output impedance is non-zero, so there is a slight voltage drop when comparing to scope image for CH1 with sig-gen settings.
7. Waveforms for the input signal, and output signal, are in attached image. There is also a video showing under same test conditions. It would seem that the opamp output is bi-stable, it switches between two very different states.

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The attached photo shows the scope when the output of the op-amp is completely different to what is expected, which causes the rest of the system to not perform as expected.

The schematic reference is "Current Measurement Ckt 2 of 3" (sent previously), but note the following changes:
1. The two capacitors C68 & C69 (0.1nF) were removed prior to this test.
2. Each opamp input (pin 5, 6) has a 1MEG ohm resistor to +12V, this biases each input to about +1V above 0VA, which is the opamp negative supply (pin 4).

The expected opamp output was drawn onto the photo using a photo-editor.

Thank you and looking forward for your kind response.

Regards,

Maynardattachment.zip

  • Hi Maynard,

    Having the information spread among several images makes understanding the issue more difficult to understand and assess the OPA2180 issue. Would it be possible for you to provide a single updated schematic with the changes you describe?

    You mention in item number 4 - Scope CH1 is connected to node "SHUNT-NEG", refer attached ckt 1 of 3, and in number 5 - Scope CH2 is connected to op-amp output, node "X42", refer attached ckt 2 of 3. When I view the scope screen image CH1 1 (orange trace ) appears to be a constant level. CH 2 and CH4 show the noisy waveforms. So is the waveform on CH1 as expected? What is the image on CH 4?

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Hi,

    I fully agree with Thomas. Many of the information given is incorrect or contradictory. No fun :-(

    Here's my guess:

    user_opa2180.TSC

    Kai