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THS4531: Current measurement for different values of resistors for a single ended to differential output circuit

Mohsin Shehzad
Intellectual 280 points
Part Number: THS4531
Other Parts Discussed in Thread: LF411

Hi Dear,

             I am using THS4531 in the quantity of 48 as the input buffer of ADCs. My input frequencies are less than 1.2MHz. The data sheet says that if RF = RG = 2 kΩ, RG to ground, and VOUT+ = 4V, 1mA current will flow through the feedback path to ground. If I increase the value of resistor the current will be decreased. 

Can you please let me know that if I go for a resistor value of 10K or 20K, how much current will flow through the feedback path to ground?  The current consumption for my application is quite critical and I want to decrease the current as much as possible.

Also, by changing the resistor value, will it effect the input of ADC, signal will be attenuated or increased?

I am anxiously waiting for the reply.

Thank you

Kind Regards:

Mohsin 

 

  • 0
    TI__Genius 15555 points

    Hello Mohsin,

    Increasing the feedback resistance will also increase the noise in your system and could possibly lower your bandwidth & affect stability if too high. Stability doesn't seem to be an issue with 10-kohm resistors for the THS4531. I've attached a TINA sim with the THS4531 using 10k-ohm resistors for evaluation. Please let me know if you have any questions. 

    Mohsin.TSC

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Intellectual 280 points

    Hi Hasan,

             Thank you, this helps.

    I have one more query. I have developed a bench circuit using an op-amp and THS4531 as single ended to differential converter using single supply (+5V and VOCM is set to 1.25V) as shown below. The problem is that when the signal level at the input of THS4531 increases, the THS4531 draws a big current from +5V supply. 

    Can you please let me know that what could be the reason? 

    I have also attached the simulation circuit.

    Thank you 

    Kind regards:

    Mohsinbench_test_circuit_THS4531.TSC

  • 0 in reply to Mohsin Shehzad
    TI__Genius 15555 points

    Seems like the single-ended output common swing of the model is greater than what is shown in the datasheet (at around 300mV from what I could tell). Is there any reason why you've set Vocm to 1.25 rather than at mid-supply? 

    Best,

    Hasan Babiker 

  • 0 in reply to Hasan Babiker31
    Intellectual 280 points

    Hi Hasan,

    Thank you.

    The ADC chip I am going to use after this has Vref = 2.5V, so I have to keep my signal around 1.25V from 0 to 2.5V.

    It is rare in actual scenario that I could get high signal level at the input of differential amplifier, but if it happens it will collapse the +5V supply, because I am using 48 differential amplifier circuits.

    Thank you

    Kind Regards

    Mohsin

  • 0 in reply to Mohsin Shehzad
    TI__Genius 15555 points

    Hey Mohsin,

    Do you expect a 2Vpp input as indicated in your sim? If the signal wasnt being attenuated because of the bandwidth of the LF411, it definetly would be exceeding the output range of the THS4531.

    Best,

    Hasan Babiker