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  • TI Thinks Resolved

THS4521: Unexpected behavior of the amplifiers within a bandpass filter

Part Number: THS4521

Hello,

I'm using the THS4521 for active 2-stage Bandpass Filter (4.14 KHz < f < 40.89 KHz), generated using FilterPRO.

This is the last part of analog receiver chain, that includes 4 stages: Instrumentation Amplifier (Gain of 300 [V/V], AD8429/Analog Devices)  -->  Single-Ended Input to Differential Output Amplifier (Gain of 2 [V/V], THS4521/TI)  -->  Active BPF (Gain of 1 [V/V], See above picture) --> A/D Converter (ads1274/TI).

In order to study the source of the noisy input to the ADC, the inputs of the Instrumentation Amplifier (INA) were shortened to ground.

The measured output signal of the INA is 30 [mVpp] (see F2 in picture #1), and it rises to 100 [mVpp;18 MHz] at the output of the Differential Output Amplifier (see F1 in picture #1). This amplifier is designed with 1 [kohm] series resistor and 2 [kohm] feedback resistor.

The measured signal at the middle stage outputs of the BPF is 714 [mVpp] at frequency of 60.98 [MHz] (see F2 in picture #2), and the signal at the last output of the BPF is 120 [mVpp] at frequency of 16.34 [MHz] (see F1 in picture #2).

 

In short - the BPF has large output noises at frequencies that do not appear anywhere at the circuit, and should be attenuated.

 

I suspect that the noisy output of the BPF is due to Bias currents of the THS4521, which are generated by the presence of the AC-Coupling on the inputs of the Op-Amps. Does it make sense?

Should the Op-Amps inputs have reference to ground, using large pull–down resistors?

 Moreover, since I didn’t see any application using resistors with large values, I changed the feedback resistors of the first stage to 10 [kohm]. No improvement was observed in the results.

Best regards,

Avihay.

P.S. 

If the BPF picture is not clear, I can send it to your email.

  • Hello Avihay,

    I did have problems reading the schematic.  You can email me a copy at loren dot siebert at ti dot com.  The capacitor C159 should have some resistance between it and the amplifier output.  I see that there are place holders for resistors, you could try using 25 Ohm capacitors as a start.  I can simulate your circuit once I have a better copy of the schematic.

    Regards,

    Loren

      Every day you are alive is another opportunity to make the world a better place. 

  • In reply to Loren Siebert 1:

    Hey Loren,

    A copy of the schematic is sent to you by email.

    I hope to hear from you soon.

    Best regards,

    Avihay.

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