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A strange power consumption of THS4531

Yu-Hung
Expert 1130 points
Other Parts Discussed in Thread: THS4531

Hi,

I made a PCB board for test ADC.

Here are the pre-amplifier schematic and THS4531 circuit soldered only.

A1+ and A1-  is connected by 1.6V battery and to measure V1IN+ and V1IN- by meter.

AVDD = 2.5V, AVSS = -2.5V are generated by power supply.

CASE 1: <C47 exist, tested in C47= 1nF or 0.1uF>

When power on, the current of AVDD is 17mA and the current of AVSS is 16mA. (shown in power supply display)

After few seconds or I touch the wire of battery holder, the current grows up to 37mA and never drop down anymore.

CASE 1: <C47 removed>

When power on, the current of AVDD is 0mA and the current of AVSS is 0mA. (shown in power supply display)

My power supply has 3 digits after decimal point, so the current maybe less than 1mA.

In CASE 1 and 2,

The outputs of THS4531 are the same (1.6V*0.125=0.2V).

So I think the circuit is work, but I have no idea why the current is so different.

@_@?

Yu-Hung

  • 0
    TI__Genius 12631 points
    Yu,
    1nF is a lot of load capacitance and is probably causing the amplifier to oscillate. Please refer to Figure 52 in the datasheet. The outputs of the amplifier will need to be isolated from the capacitive load by using a 10ohm resistor (for 1nF Cload).
    -Samir
  • 0 in reply to Samir Cherian
    Expert 1130 points

    Hello Samir,

    It works after I put 2 small resistors.

    Thanks. :)

    Is it fine If I put a LPF here?

    The resistors is larger than 10ohm.

    YHH

  • 0 in reply to Yu-Hung
    TI__Genius 12631 points
    That is a large resistor you are using. If you would like to use it as a filter then that is certainly an option. Please be mindful that the 8k resistor will have some noise. Also, if the following stage is an ADC look for the input resistance of the ADC since the 8k resistors will form a voltage divider circuit and attenuate your signal output. Is the BW of your circuit only 200 Hz?
  • 0 in reply to Samir Cherian
    Expert 1130 points

    Hi Samir,

    Thanks for your salutary reminder.

    I want to measure the signal in low sampling rate, approximately 100Hz.

    In the TI-Tina simulation, this filter also has a bad phase response.

    Pre-amplifier.TSC

    But the previous design (it is also a LPF) can't get such low frequency.

    Maybe forget it :)

  • 0 in reply to Yu-Hung
    TI__Genius 12631 points
    Hi,
    I just checked the magnitude response and it looks okay. The phase response will always show phase shift when the signal of interest is close to the filters cutoff frequency. Unfortunately there is no way around this. You could increase the BW of the filter to 1kHz and still reduce noise but have better phase response. This is obviously going to be a trade-off.
    -Samir
  • 0 in reply to Samir Cherian
    Expert 1130 points

    Hi Samir,

    Thanks for the hard work.

    It's really a trade-off.

    Ideal solution is not existing on the world.

    HaHa