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OPA4188: problems in using OPA4188

Part Number: OPA4188
Other Parts Discussed in Thread: LM224, OPA4196

Hi team,

There are some problems in using OPA4188.

The triangle wave generator composed of lm244 in the original design circuit, but it is easily affected by temperature and the signal is unstable. So the zero drift op amp OPA4188 is used instead. This chip and LM244 are the same 4-channel op amp with the same pin and package. but the output waveform and frequency are not correct after replacement.

The original circuit is as follows:

U1B is composed of square wave circuit, U1C is composed of triangle wave circuit, U1A and U1D are composed of comparator. The sampling values of V1 and V2 voltage are compared with the output voltage of triangle wave respectively, and the PWM waveform is output to convert the voltage signal into PWM signal.

In addition:after our analysis, does the 4188 op amp have input clamp, which makes the output incorrect?
Or is there any other low drift op amp suitable for our circuit.

Thanks a lot.

  • Hi Xu,

    By all accounts the OPA4188 is an electrically superior replacement for the legacy LM224 ( I think you meant LM224, LM244 is a non-TI LED). However, because they are so different in design and electrical performances from each other that could explain the circuit performance differences being observed.

    Yes, the OPA4188 has back-to-back input clamping diodes. The LM224 uses a vertical PNP differential input pair that has a high reverse emitter-base breakdown voltage and doesn't require the input clamping diodes.

    I suggest trying one of the precision e-trim op amps such as the OPA4196. They do not have the back-to-back input clamps - just in case that is a factor in defining the circuit's operation.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • Hi Zhonghui,

    I agree with Thomas. Anti-parallel input protection diodes have an impact on the oscillator performance: