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Original question:

PGA970: Hardware design issue with PGA970

PGA970: LPF between PI and PE

Martin Wunderlich
Prodigy 150 points
Part Number: PGA970

Dear TI-Team, 

I'm creating my first circuit diagram for the PGA970 and just got stuck at the low-pass filter between pins PI and PE. Figure 19 in the PGA970 datasheet (slds201a.pdf) suggests that the input impedance of the waveform-gain block cannot be neglected and the resistor of the external LPF would alter the gain. 

I could not find information on the input resistance of the waveform-gain amplifier. In the PGA970 EVM the LPF is a placement option with a default 0R00 between PI and PE. Is a passive LPF recommended or would you suggest the direct connection between PI and PE? Or maybe an active LPF? 

Kind regards, 

Martin

  • 0
    TI__Mastermind 31655 points

    Hello Martin,

    I have mainly seen a passive LPF recommended between PI and PE. It is curious that the input impedance of the waveform gain amplifier is not specified, so I will look into that for you. I'm not sure if the input structure to the amplifier is exactly as shown in the block diagram or if there is more in place to reduce the effects of additional input impedance.

    In the meantime I would recommend choosing a small resistance for your LPF. What would you like the corner frequency of your LPF to be set at?


    Regards,

  • 0 in reply to Scott Cummins
    Prodigy 150 points

    Hello Scott, 

    I also had the notion that figure 19 from the datasheet might just be  coarse schematic diagram. However, I wanted to check that before I make my design. As far as I understood, the purpose of that LPF is to attenuate the 1 MHz sampling frequency of the waveform DAC and to smoothen the excitation sine wave. Our LVDT will need an excitation frequency in the range of 3 kHz to 5 kHz. My idea at the moment is to make the LPF 220 R / 10 nF, so the corner frequency would be around 72 kHz and it would attanuate the 1 MHz by ca. 23 dB with only minor attenuation to the excitation signal. 

    Kind regards,

    Martin

  • 0 in reply to Martin Wunderlich
    TI__Mastermind 31655 points

    Hi Martin,

    I just confirmed there is a buffer between the PE pin and the amplifier stage, so the input impedance should be in the Giga ohms range. You shouldn't have to worry about component values affecting the waveform gain.

    Regards,

  • 0 in reply to Scott Cummins
    Prodigy 150 points

    Hi Scott, 

    very good news, thank you very much!

    Kind regards,

    Martin