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BUF634A: Output offset voltage questions

Jerome Bolduc
Prodigy 50 points

Part Number: BUF634A
Other Parts Discussed in Thread: BUF602, BUF634

Hi,

I made some observations while testing a BUF634A buffer (in real life). The offset voltage between input and output signal seems to be greater when it is sinking than when it is sourcing.

For example:

-I apply a 5kHz +100mV/-100mV sinewave with a +5V DC offset to the input, with a 680 ohms load (a resistor) at the output. Result: The output sinewave has a 10mV offset.

-I apply a 5kHz +100mV/-100mV sinewave with a -5V DC offset to the input, with a 680 ohms load (a resistor) at the output. Result: The output sinewave has a 100mV offset.

What could explain that?

Also, the offset is supposed to stay the same no matter what's the value of the load right?(provided that I stay below the 250mA limit of course)

In brief I am trying to understand why there is that inconsistent offset, and also the relationship between the load and the output voltage offset.

I can explain more and provide more details.

Thanks in advance,

Jerome

  • 0
    TI__Expert 3880 points

    Hi Jerome,
    Can you confirm if the output load is connected to the ground or some other node, by that I mean is the value of the device output current the same when it's sinking (-5V DC offset to the input) and sourcing (+5V DC offset to the input)?

    The gain of the device (BUF634A) varies slightly with the output load/current. Say for example, the gain will be 0.99V/V at 1K load (Vout=10V) but drops to 0.93V/V at 67ohm load (Vout=10V) (refer to the DC performance table of the electrical characteristics). This could be a reason why you are seeing a different offset at the output.

    Regards
    Anant

  • 0
    TI__Mastermind 31915 points

    Hello Jerome,

      Agreed with Anant. Also, what are the power supplies to the device? 

    Thank you,

    Sima 

  • 0
    Guru 48395 points

    Yes, Anant, the BUF634 is an open loop buffer, so it has a relatively high output impedance - that becomes part of the gain equation into your load and will look like offset voltage but is probably more like a gain error. The BUF602 is closed loop, but may not have the swing or Io you need. 

  • 0 in reply to Michael Steffes
    Guru 48395 points

    Yes, the BUF634A specs a typ 5ohm output impedance, so that needs to be considered in the output response to a heavy load. If you need better accuracy and/or offset - use an op amp in unity gain