Programmable Gain Transimpedance Amplifiers Maximize Dynamic Range of Spectroscopy Systems (IV)

Programmable Gain Transimpedance Amplifier

　　Figure 9 shows a programmable gain transimpedance amplifier. This is a good conceptual design, but the on-resistance and leakage current of the analog switches introduce errors. The on-resistance causes voltage- and temperature-dependent gain errors, and the leakage current causes offset errors, especially at high temperatures.

　　Figure 9. Programmable transimpedance amplifier.

　　The circuit shown in Figure 10 avoids this problem by using two switches in each transimpedance branch. Although it requires twice the number of switches, the on-resistance of the left switch is within the feedback loop, so the output voltage depends only on the current through the selected resistor. The right switch appears as an output impedance, which produces negligible error if the amplifier is driving a high impedance load such as an ADC driver.

　　Figure 10. Programmable gain transimpedance amplifier with Kelvin switch.

　　The circuit of Figure 10 is suitable for DC and low frequencies, but in the off state, parasitic capacitances on the switches are another problem. These parasitic capacitances, marked as Cp in Figure 10, connect the unused feedback path to the output, thereby reducing the overall bandwidth. Figure 11 shows how these capacitances are ultimately connected to the unselected gain branches, thus changing the transimpedance gain to a parallel combination of the selected gain and an attenuated version of the unselected gain.

　　Figure 11. Total feedback capacitance including switch parasitic capacitance.

　　Depending on the desired bandwidth and feedback resistors, parasitic capacitance can cause the expected behavior of an amplifier to differ significantly from the measured behavior. For example, assume the amplifier in Figure 11 uses the same 1 MΩ and 10 MΩ values ​​as the previous circuit, with corresponding capacitances of 4.7 pF and 0.47 pF, respectively, and we choose a gain of 10 MΩ. If each switch has a feedthrough capacitance of approximately 0.5 pF, the difference between the ideal bandwidth and the actual bandwidth, taking into account parasitic paths, is shown in Figure 12.

　　Figure 12. Transimpedance gain including parasitic switch capacitance.

　　One way to solve this problem is to replace each switch with two switches in series. This will halve the parasitic capacitance, but will require more components. Figure 13 shows this approach.

　　Figure 13. Adding series switches to reduce total parasitic capacitance.

　　If the application requires higher bandwidth, a third approach is to connect each unused input to ground using an SPDT switch. Although the parasitic capacitance of each open switch is still in the circuit, Figure 14b shows how each parasitic capacitance appears to be connected from the output of the op amp to ground, or from the end of the unused feedback branch to ground. Capacitance from the amplifier output to ground often causes circuit instability and ringing, but in this case the total parasitic capacitance is only a few pF and will not have a serious impact on the output. The parasitic capacitance from the inverting input to ground will add the shunt capacitance of the photodiode and the op amp's own input capacitance, and the increase is negligible compared to the large shunt capacitance of the photodiode. Assuming a 0.5 pF feedthrough capacitance for each switch, this will add a 2 pF load to the op amp output, which most op amps can drive without difficulty.

　　Figure 14. Programmable TIA using SPDT switches.

　　However, like anything, the approach shown in Figure 14 has disadvantages. It is more complex and can be difficult to implement for gains above two. In addition, the two switches in the feedback loop introduce dc errors and distortion. Depending on the value of the feedback resistor, the extra bandwidth may be significant enough to ensure that such small errors do not affect the circuit operation. For example, with a 1 MΩ feedback resistor, the on resistance of the ADG633 produces approximately 50 ppm of gain error and 5 μV of offset error at room temperature. However, if the application requires the highest bandwidth, then this can be considered a disadvantage.

　　in conclusion

　　Photodiode amplifiers are an essential component of most chemical analysis and material identification signal chains. Using programmable gain, engineers can design instruments to accurately measure very large dynamic ranges. This article shows how to achieve high bandwidth and low noise while ensuring stability. Designing a programmable gain TIA involves challenges such as switch configuration, parasitic capacitance, leakage current, and distortion, but choosing the right configuration and carefully weighing the trade-offs can achieve excellent performance.