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  • TI Thinks Resolved

OPA350: OPA350

Prodigy 10 points

Replies: 4

Views: 68

Part Number: OPA350

Hi. I am referring to the following remark in the forum (Sep 7, 2017) about using an OPA350 in a photodiode transimpedance amplifier:  

"Also, please be aware that since OPA350 output can only swing linearly within 50mV above its negative rail, under the dark current condition (no current in diode) for the circuit to work properly, you may NOT power the circuit from a single supply."

My questions: Why is that so and what would be a proper/advisable power supply, for example Vcc- -1V and Vcc+ +4V ?

Thanks,

Folkert

  • Yes, most of these RRO devices need some small headroom to operate close to ground on the output in single supply, the LM7705 was intended to provide that small negative supply

    Michael Steffes

  • In reply to Michael Steffes:

    Hi Folkert,

    My questions: Why is that so and what would be a proper/advisable power supply, for example Vcc- -1V and Vcc+ +4V ?

    Answer:


    There are no big advantages to place wider voltage rails in OPA350, if you want to keep Vout/Iin operating in linear region. I will recommend to keep V- slightly negative biased (-0.5 to -1V or so) and V+ is what is available on your board with  V+ < 5.5V, see the captured image below. 

    I think that there is a "typo" in the Input common mode Voltage range, which it should have been (V-)-0.1 rather than -0.1V. However, the section 6.6 in the datasheet is using the single voltage rail when the data was obtained, therefore -0.1V is also correct in that content, since V- = 0 = GND. 

    Best,

    Raymond

  • In reply to Raymond Zhang1:

    Also Folkert,this comes up so often it was the 1st topic in this high speed amplifier series I am doing on Planet Analog, 

    https://www.planetanalog.com/input-and-output-voltage-range-issues-for-high-speed-amplifiers-insight-1/

    Michael Steffes

  • The simplest solution would be to use a single 5V supply and drive the negative supply with LM7705 (-230mV negative charge pump).

    Marek Lis, MGTS
    Sr Application Engineer
    Precision Analog - TI Tucson

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