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PGA870EVM: How to make the board fully-differential? Also output impedance?

Zolboo Byambadorj
Prodigy 30 points
Part Number: PGA870EVM
Other Parts Discussed in Thread: PGA870

How can I modify the board to make it fully-differential? Because default circuit has negative input and output are grounded.

I tried to remove R2 and R17 and put 0 Ohm resistor at R20. In result, it seems fully-differential but output signal didn't amplified when I set 20dB (signal generator has 50Ohm outputs and oscilloscope also has 50Ohm inputs). Also, how much is the output impedance of the board? I can't find it on datasheet of evaluation board.

Thanks.

  • 0
    TI__Expert 8400 points

    Hi, 

    You have the right resistor configuration to make the part differential input and differential output. The output impedance is going to be 242 Ohms in your configuration based on the standard resistors on the board. The actual amplifier output impedance is negligible at approximately 3 ohms. 

    Regards, 

  • 0 in reply to Jacob Freet
    Prodigy 30 points

    Hello Mr. Jacob,

    Thank you very much for you response.

    I'd like to ask how can I match the impedance of input and output of the evaluation board to differential 100 Ohm.

    Could you explain how the output 242 Ohm calculated? Also what would be the imput impedance?

    Best regards,

  • 0 in reply to Zolboo Byambadorj
    TI__Expert 8400 points

    Hi, 

    The 242 Ohms is calculated from adding the resistors R11, R12, R14, and R15. With the additional 3.5 Ohms from the output impedance of the part, the total differential would be approximately 245 ohms. 

    To make the part 100 Ohms differential output, you just need to change the value of the resistors. The easiest way is probably to just remove R14 and R15, and then replace R11 and R12 with 49 Ohm resistors. That will give you a total differential output impedance of approximately 101 Ohms. The transformer on the board is a 1:1 impedance ratio, so you should then see 101 Ohms differential at the output. 

    Regards, 

  • 0 in reply to Jacob Freet
    Prodigy 30 points

    Hello Mr. Jacob,

    Thank you very much for the response.

    The output impedance is now match.

    Could you tell me how can I calculated input impedance of the board? Is it a sum of R6,R7, R8, R10 and 282 Ohm (input resistance of IC)?

    And how can I match the input impedance to differential 100 Ohm?

    Thanks in advance.

    Best regards,

  • 0 in reply to Zolboo Byambadorj
    TI__Expert 8400 points

    Hi, 

    The differential input impedance of the PGA870 is 150 Ohms, which will look like 150 ohms between the two inputs. In order to make this look like 100 Ohms, you will need to put a parallel resistance across the inputs that creates an equivalent 100 ohms total input differential impedance. Using the EVM, you can replace R6 and R7 with 150 ohm resistors. That will give you an input impedance of 300 Ohms in parallel with 150 ohms, which equals 100 ohms. 

    You can leave the series resistors as 0 Ohm shorts and remove resistor R2 to get a fully differential input. 

    Regards,