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THS4509 with single supply on the ADS4226EVM

Zhihua Liao
Prodigy 140 points
Other Parts Discussed in Thread: THS4509, ADS4226EVM, ADS4226, THS4508

Hi,

The default analog input configuration is a transformer coupling through T1 and T2 for channel A, and T3 and T4 for channel B. There is an optional configuration for analog input is through an Operational-Amplifier (OPA), THS4509 on the ADS4226EVM.

I want to use the THS4509 for analog input, but the supply for THS4509 is dual supply.then, if it can be changed to single supply for THS4509 what should be done. In other words, which components should be installed except the component have  illustrated in the  Table 3 from SLAU333A  document and the value of component should be changed. 

Analog input from the AIN_CH-A J6, the range of signal is 1uv~10mV and the bandwidth is 125Mhz. The signal amplifies to 2-Vpp through THS4509, how to calculate the value for resistor relate to OPA-Driven Input circuit.

Schematic would be better for me.  I really appreciate it.

  • 0
    TI__Genius 13335 points

    Hello Zhihua,

    Although the THS4509 can be used in a single supply configuration, I do not believe it is appropriate in this case. The THS4509 is not rail-to-rail amplifier and therefore requires split supply to keep the output centered to the ADC common mode. The output is limited to (Vs-) + 1.1V on the low side. Given the absolute maximum restriction of 1.9V on AVDD you would end up wasting most of the ADC’s dynamic range and realistically end up clipping the majority of the signal. One alternative is to use a THS4508, which is similar and pin for pin compatible with the THS4509 but the common mode range is shifted to -0.3V to 2.3V. This would give much better performance for driving the ADS4226 single supply. AC coupling is another option.

    The second problem is the large amplitude range of the input. If you want 10mVpp to correspond to 2Vpp then you are talking about a gain of 200V/V. The problem comes when you amplify 1uV, the result is only 0.2mV. The ADC resolution is 12 bits, so on a 2V full scale range the ADC will only be able to resolve 0.5mV meaning the signal would be lost in the noise floor. This doesn't take into account ENOB or amplifier noise either. Additionally, the GBP required to amplify by 200V/V @ 125MHz is 25GHz which is well beyond the range of any amplifier. You may be able to cascade a couple of amps (20V/V first stage, 10V/V second stage for example) to reduce this requirement, but then you would run into problems with amplifying noise and losing accuracy for low amplitudes. Also, the EVM does not support this condition so you would need to build a new PCB. However, if you decide to go this route we offer the design files on the product page as a starting point: http://www.ti.com/litv/zip/slac459b

    Hopefully this helps answer you questions.