SBOA521 June   2021 OPA3S328

 

  1.   Trademarks
  2. 1Introduction
  3. 2The Transimpedance Amplifier and Photodiode Sensor
  4. 3The Programmable Gain Transimpedance Amplifier
  5. 4Stability of the Transimpedance Amplifier
  6. 5Integrated Switch Characteristics
    1. 5.1 TIA Stability and Switch Capacitance
    2. 5.2 TIA Output Swing and Switch On-Resistance
    3. 5.3 TIA Gain Error due to Switch On-Resistance
  7. 6Frequency Response Simulations
  8. 7Conclusion
  9. 8References

5 Integrated Switch Characteristics

The OPA3S328 incorporates two sets of low leakage, low capacitance and low on-resistance switches useful for different circuit configurations, including programmable gain transimpedance applications. The QFN package version of the OPA3S328 device features switch 1 in a 1:2 matrix configuration, one input (INSA) with two outputs (OUTSA1, 2) and switch 2, in a 1:3 matrix configuration, one input (INSB) with three outputs (OUTSB1, 2, 3). On this circuit example, switch 1 is used to build the programmable gain TIA.

Table 5-1 shows key switch parameters based on the OPA3S328 data sheet specifications:

Table 5-1 Integrated Switches Characteristics
Parameter Test Condition Min TYP Max Unit
CIN Switch input capacitance Switch open, INSA/B = 2.5 V 2.3 pF
COUT Switch output capacitance Switch open, OUTSA/B/1/2/3 = 2.5 V 0.7 pF
CINOUT Switch total capacitance Switch closed, INSA/B = OUTSA/B/1/2/3 = 2.5 V 6 pF
RON Switch on resistance Switch closed, V+ = 5 V, INSA/B = 2.5 V 90 125 Ω

CIN and COUT represent the open switch parasitic switch capacitance, while CINOUT represents the total switch capacitance while the switch is closed. RON is the switch series resistance when the switch is closed. Figure 5-1 shows a simplified model based on the switch data sheet parameters used in the TINA simulations using the switch input and output capacitances and the switch on resistance:

GUID-20210604-CA0I-KVGW-JVWS-3KQTKJMKJKV8-low.gifFigure 5-1 OPA3S328 Simplified Switch Model

When designing the switch gain transimpedance amplifier, the designer needs to consider the non-ideal characteristics of the switches. The bandwidth and stability of the transimpedance will be affected by the capacitances and resistances of the switch. Furthermore, the on-resistance RON of the switch can be a source of error on the gain of the transimpedance amplifier. The following sections discuss errors due to the switch non-ideal behavior and proposes circuits to overcome these errors.