ZHCSBX2D December 2013 – August 2016 OPA857
PRODUCTION DATA.
NOTE
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The OPA857 is a transimpedance amplifier offering two selectable gains. This device is used in conjunction with a photodiode at its input. The output is pseudo differential and may or may not require the use of a fully differential amplifier, depending on the analog-to-digital converter (ADC) used for implementation.
The OPA857 requires a photodiode to be connected to the positive bias voltage because the output voltage can only swing down from the reference voltage (1.85 V for a 3.3-V supply) to ground.
Figure 39 presents a complete end-to-end receive signal chain for an optical input. It includes a high-speed photodiode, the OPA857, a THS4541 fully-differential amplifier, and a 16-bit, 160-MSPS, high-speed ADC. For the complete wide-bandwidth, optical front-end reference design, go to http://www.ti.com/tool/TIDA-00725.
For this example, use the values listed in Table 3 for the input parameters.
DESIGN PARAMETER | EXAMPLE VALUE |
---|---|
Supply voltage | 5-V external supply |
Analog bandwidth | 120 MHz |
ADC sampling rate | 160 MSPS |
Maximum system gain | 100 kΩ |
Programmable transimpedance gain | 5 kΩ / 20 kΩ |
Maximum signal swing | 1 VPP |
Noise performance | ≥ 60-dB SNR |
Averaged noise performance | < 10-µVRMS |
TZ Gain = 20 kΩ |
TZ Gain = 5 kΩ |
TZ Gain = 20 kΩ |
TZ Gain = 5 kΩ |
At the core of the OPA857 is an ultrawide bandwidth op amp. One of the highlights of the OPA857 is the relatively small change in the transimpedance bandwidth as a function of the internal gain selected; 130 MHz (gain = 5 kΩ) and 105 MHz (gain = 20 kΩ). Theoretically, for a four times increase in gain, the bandwidth should reduce by two times; however, as observed in the case of the OPA857, the results do not follow theory. For more information on the various factors that contribute to an amplifier frequency-response performance when configured as a TIA, see What You Need To Know About Transimpedance Amplifiers – Part 1 on the TI E2E Community website at e2e.ti.com. This blog also contains a reference to an excel calculator to simplify TIA designs when using discrete opamps. The OPA857 is unique in displaying this type of behavior because the CTRL logic controls an internal switch in the amplifier core that recompensates the amplifier open-loop gain characteristic depending upon the logic level. In this application, it it shown how the closed-loop transimpedance bandwidth can be increased to greater than 250 MHz. The circuit used for this test is shown in Figure 44. An external feedback resistor, RF, is added in parallel to the internal transimpedance gain resistors of the OPA857. This resistor has the effect of reducing the overall transimpedance gain, but with increased bandwidth.
For this example, use the values listed in Table 4 for the input parameters.
DESIGN PARAMETER | EXAMPLE VALUE |
---|---|
Supply voltage | 3.3 V |
Output swing | 500 mVPP |
Differential output load | 500 kΩ and 1 kΩ |
Target bandwidth | 250 MHz |
Effective transimpedance gain | 5 kΩ |
Figure 45 shows the frequency response with a feedback resistance of 6.8 kΩ and an output load of 500 Ω. The large amount of peaking indicates a low phase-margin and potential instability. Next, a 0.1-pF feedback capacitor, CF, is added in parallel to the 6.8-kΩ RF. Both RF and CF interact to create pole in the noise gain curve that counteracts the effect of the zero caused by RF, and the total input capacitance at pin IN of the OPA857. The input capacitance is caused by the opamps inherent input capacitance, the photodiode capacitance, and the parasitic input capacitance from the PCB. The pole zero cancellation increases the phase margin, as is evident in the reduced peaking shown in Figure 46. In Figure 47, an output load of 1 kΩ was used, along with an RF = 6.8 kΩ and CF = 0.1 pF. The reduced load helps to increase the op amp open-loop gain, which in turn increases the closed-loop bandwidth of the OPA857 circuit.
RF = 6.8 kΩ |
RF = 6.8 kΩ, CF = 0.1 pF |
RF = 6.8 kΩ, CF = 0.1 pF |