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Original question:

TLE2141: Scaling and biasing a.c. signal for ADC input

TL431: Adjustable voltage reference that can sink or source up to 0.25mA

D B17
Intellectual 260 points
Other Parts Discussed in Thread: TL431, REF3425, REF5025, OPA375

Hello

I need a stable, low noise voltage reference that I can adjust between 1.4V and 1.8V. The output should source or sink up to 0.25mA @150Hz max

My proposed solution is below. Two questions:

1. The +5V, -5V supply comes from a linear power supply. Where should I put a decoupling capacitor on the design? Between the non-inverting input and ground? Between the TL431 anode and cathode?

2. The TLE2141IP consumes over 5mA of current. This is a lot just to supply 0.25mA max. load current !   Is there an alternative, stable, low noise op amp that you could recommend please?

Thank you.

  • 0
    TI__Mastermind 19370 points
    Hi,

    If you are looking for the lowest noise setup, I would not use a TL431 as it is a noisy device relative to other voltage references. The main source of the reference voltage should come from either a REF3425 or a REF5025. These 2 devices have our lowest output noise. The output can then be divided down like you have in the added schematic. Since the reference voltage is being buffered, then I would also suggest to add an RC filter to further reduce the noise.

    In regards for low noise op amps, the main rule of thumb is low noise requires more power. It is difficult to get a low noise op amp at low power. Something like the OPA375 has low noise and only consumes a low Iq of 1mA. The OPA375 datasheet also shows a low pass filter schematic that can help further reduce noise.

    -Marcoo
  • 0 in reply to Marcoo Z
    Intellectual 260 points

    Thanks Marcoo.

    My frequency range of interest is 0-150Hz and I'm wondering if, in this range, there is much difference between the REF3425 and TL431 with regards to noise?

    I attach extracts from the REF3425 datasheet (left) and TL431 datasheet right (right). Are the two devices very different at low frequency, or am I misunderstanding the datasheets? Both devices seem to emit +/-2microvolt noise, with occasional peaks outside this range.

    I can't find an equivalent graph in the datasheet for LM4040DIZ-2.5/NOPB, which I suppose is another alternative?

  • 0 in reply to D B17
    TI__Mastermind 19370 points
    Hi D B17,

    the reason I bring up a device like the REF3425 is that the TL431 is biased at 10mA to achieve noise levels that low. If you are ok with that level of power being used then there is no issue. The other concern is that the accuracy and temperature coefficient will need to be calibrated for each device just to ensure that the gain error of the TL431 does not affect the measurement and the majority of the error is from the noise. Without calibration the initial accuracy error and temp co will dominate any error from the noise.

    -Marcoo
  • 0 in reply to Marcoo Z
    Intellectual 260 points

    Understood. Thanks Marcoo. I plan to calibrate the device so this is no problem. Appreciate your support.