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LMH6629: Application note SNOA942 clarifications

Mahesh KC03
Intellectual 1360 points
Part Number: LMH6629
Other Parts Discussed in Thread: TINA-TI

Hi,

One of my customers has the below questions regarding the Transimpedance amplifers. 

1) In application note SNOA942 "Transimpedance Amplifiers (TIA): Choosing the Best Amplifier for the Job", a design example for photodiode is given (section 3). They have proposed LMH6629 opamp having 15uA bias current for interfacing a photodiode having a maximum current output of 1uA. How can this be justified?

2) If I am using an opamp having GBP of 2 MHz, the maximum Transimpedance gain I can give for current to voltage conversion of a 10KHz signal is only 200 ?.

3) Once the current is converted to voltage using a TIA whether the following preamplifier bias/offset currents have strong impact on the performance of the channel.i.e.Whether using opamps having bias currents in uAmperes for amplification/filtering after the TIA (for converting uA

currents) is a good design practise.

application note SNOA942 

  • 0
    TI__Genius 12631 points

    Mahesh,

    1.  The input bias current will only create a dc offset. The 1uA signal will ride on top of the bias offset. You can also have an equal size resistor on the noninverting pin to cancel the bias current.

    2. If I am using an opamp having GBP of 2 MHz, the maximum Transimpedance gain I can give for current to voltage conversion of a 10KHz signal is only 200 ? - That is not true, you are confusing transimpedance gain with noise gain. The noise gain will affect the amplifiers gain bandwidth product. There are a couple of papers you can read on understanding the relationship between Transimpedance Gain and how it affects the amplifier bandwidth.

    3. The bias currents of the following stages will not affect the TIA output current, but bias currents get multiplied by resistances to create offset voltages. If care is no taken the consequent stages can very well affect the overall accuracy and SNR of the system. However if care is taken to optimize the TIA stage and maximizing its SNR the consequent stages will have a reduced affect.

    Attached is some reading for TIAs

    www.ti.com/.../sboa122.pdf

    e2e.ti.com/.../what-you-need-to-know-about-transimpedance-amplifiers-part-1

    e2e.ti.com/.../what-you-need-to-know-about-transimpedance-amplifiers-part-2

    -Samir

  • 0 in reply to Samir Cherian
    TI__Intellectual 1360 points
    Thanks Samir.
    According to my understanding, the Input bias current, if directed inwards, comes from the signal source. If the maximum current from the source is 1uA only and the bias current of op-amp is 10uA, would not the op-amp load the photo-diode?
    Is my understanding correct?

    I have typically seen customers demand an op-amp with very low bias current for TIA and Accelerometer applications.
  • 0 in reply to Mahesh KC03
    TI__Genius 12631 points

    The opamp would not load the PD. The current for the Ib of the opamp would come from the opamp output. This would therefore create an output referred offset. If the application is precision based or if you have a very high feedback resistance then the JFET/CMOS input opamp would be the right choice (small bias current and low current noise).

    If however the application requires very high-speed and the feedback resistance is < 10kOhm, then the BJT amplifier may be the better choice. Check out this app note on TIAs for a better understanding.

    The choice of amplifier really depends on the specific application needs. Please also use TINA-TI to simulate noise/offsets/bandwidth etc. This should help you get a better understanding of the TIA.

    -Samir

    www.ti.com/.../sboa122.pdf