How to build a TIA circuit using fully differential amplifiers

Trans-impedance amplifiers (TIA) are most often built using operational amplifiers (op amps).  And, more and more, if not all, analog to digital converters (ADC) are fully differential systems required to have a single-ended differential mechanism.  For applications requiring DC coupling, this is mostly achieved using fully differential amplifiers (FDA). 

Is there a way to use a FDA for the I-to-V conversion, as well as directly interface to the ADC?  In short, the answer is YES and here we’ll review implementation and limitations.

Prior to developing this further, I need to warn you about the gain limitation of using an FDA as a TIA.  The input bias current of the bipolar input stage will in effect limit the maximum gain achievable in a transimpedance stage.  You can expect the input bias current to be several uA.  In op amps, if both the input bias current and the feedback resistor are large it will create an offset on the output. In FDAs, the input bias current will create a common-mode offset.  This is less of a problem since the output voltage swing is double that of an op-amp with the FDA’s output stage, but it will need to be kept in check for higher transimpedance gains.

Let’s start by looking at the OPA843, a bipolar amplifier with excellent noise and gain-bandwidth set for a gain of 20kW and using a 1.5pF input capacitance for the photodiode.  Under these conditions, the OPA843 can achieve a flat frequency response to ~57MHz.  See figure 1 below.

                                                               Figure 1: OPA843 TIA configuration

The performances of interest we will be monitoring are:

1-      Small signal frequency response with its associated flatness to ensure no ringing in the pulse response as well as well behaved phase variation.

2-      Integrated noise on the output.

3-      Power dissipation of the solution.

Figure 2 and figure 3 shows the OPA843 performance monitored here.

                                                            Figure 2: OPA843 Frequency response

                                                                 Figure 3: OPA843 Integrated output noise

The OPA843 power consumption is ~200mW when operating on ±5V supply.

Looking at the 14mA on +5V supply FDA (70mW) THS4520, we can achieve the same 20kΩ transimpedance gain by using 10kΩ resistor in the feedback path.  For this gain to be 20kΩ, the presence of C4 is required.  Note that for frequency below the pole formed by R2 and C4 (16Hz here), the gain is reduced by 6dB.

                                                                       Figure 4: THS4520 TIA configuration

The THS4520 achieves ~74MHz bandwidth with much lower integrated noise than the OPA843, despite the increase bandwidth.  This reduced noise is due in part to the lower current noise density (same voltage noise density) of the THS4520, but also due to the lower feedback resistor used to achieve the same gain and the higher compensation feedback capacitor leading to lower noise gain.  The two dominant terms for the noise reduction are the lower noise gain vs. frequency and the lower feedback resistor with its associated thermal noise.  The results are plotted in figure 5 and figure 6.

                                                                Figure 5: THS4520 Frequency Response

                                                                     Figure 6: THS4520 Integrated output noise

Where else have you applied an FDA where a traditional op amp is used?

  • Thanks for this article.

    Short and clear.

  • Dear

    it's a worthy idea and I simulated it . the result was very good.

    but consider we want to put a TIA amplifier far away from ADC driver (for example 1meter). now how we should transmit the differential output signal of THS4520 to the ADC driver? with a twisted pair cable?

    I mean most of twisted pair cables have 4 STP like CAT5, CAT6, CAT7. they are not flexible and  I think they are suitable for such application.

    but with OPA843 or better Op-Amps it's enough to use a 50Ohm coaxial cable.

    golhaaa@gmail.com

    Regards