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ADS5294: Need differencial input?

Stefan Thorlacius
Intellectual 570 points

Part Number: ADS5294
Other Parts Discussed in Thread: THS4541

Dear Sir or Madame
We intend to use a couple of ADS5294 in our design where we want to measure the back voltage over a photo diod.
We are looking into ADS5294, due to LVDS interface and 8 pipelined ADC and at least 20 MSPS.
http://www.ti.com/lit/ds/symlink/ads5294.pdf
In our case we must use the BPW 34-S
media.osram.info/.../BPW 34 S E9601.pdf
and will connect it "backward" to VCC via a resistor, the idea is to let the ADS5294 measure the volatage changes over the photodiod.
This mean that the voltage range will be VCC down to whatever the photodiod will lower "itself to", 
do we really need any impedance adaption here?
According to the reference design
www.ti.com/.../slau355a.pdf
There is a lot of components between the analouge input pin and the ADS5294, are they neccessary for our application
or can we connect the photo diode directly to the ADS5294?
That is, IN1P to photo diod and IN1N directly to ground (sort of single ended coupling)?
Or do we, at least connect an op in between, like the High-Speed Differential I/O Amplifier THS4541?
Regards Stefan

  • 0
    TI__Expert 6040 points

    Thanks for considering the ADS5294!

    Our EVM design has a lot of options for different configurations! so you see quite bit components. in the ADS5294 datasheet, we have common circuits figure 62 ias a n example. 

    we recommend to have a transimpedance amplifier between ADC and photodiode. as i understand, most photodiodes are current source. Thus directlly connected to ADC might be tricky. THS4541 is a good opamp options. 

    if you use a I/V converter by Resistor, the driving capability could be an concern. our ADC has internal 2K ohm for VCM bias. so your I/V resistor may become a  voltage divider. 

    if you use as singled end operation, it will reduce the input range from 2Vpp to 1Vp. also other noise spec and HDx spec will be worse, due to unbanlanced input. 

    Thanks!