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MSP430FR2310: SAC OpAmp's Input Impedance

Victor Salomon
Expert 3540 points
Part Number: MSP430FR2310
Other Parts Discussed in Thread: OPA1692, TLV313, OPA388

Hello team,

I am looking to try to figure out the Input Impedance (ZIN) for the SAC OpAmp – looking at MSP430FR2310’s Table 5-24 on the Datasheet (SLASE58) I found the ‘Common Mode’ Input Capacitance (CIN) specified as 2.0 pF.

Q1: What is the ‘Differential’ Input Capacitance (CIN)?
Q2: What is the ‘Common Mode’ and ‘Differential’ Input Resistance (RIN)?

Ideally, I’d like to see something similar what is typically specified in OpAmps’ Datasheets, for example, OPA1692’s Datasheet on Section 6.5 (SBOS566) 

  • 0
    TI__Expert 7530 points
    Hi Victor,

    Good questions here. The SAC op-amps and many other CMOS op-amps do not always have an input resistance specification but rather just an input bias current specification, as the input resistance is quite high when compared with a bipolar op-amp. For example, take a look at the datasheet for a TLV313 and you'll see specs for input bias but not input resistance. Thus it is difficult to directly compare the specs of a bipolar OA such as the OPA1692 with a CMOS OA architecture.

    That being said, I do agree that the input capacitance values that are specified in the MSP430FR2310 datasheet lack the differential input capacitance values.

    May I ask the nature of the application in question? We can work do address any shortcomings in the spec as it relates to a specific application if needed.

    Regards,
    Walter
  • 0 in reply to Walter Schnoor
    TI__Expert 3540 points

    Walter,

    I know that come CMOS OpAmps such as OPA388 do specify the Input Resistance, but I understand the resistance is typically higher in CMOS input stages it might not be specified.

    I want to drive a voltage divider into the SAC OpAmp so I was trying to figure out how high of resistor values could I possibly use – but if it is on the 100's of MΩ or higher it really isn't much of a concern.

    It would be helpful to know Differential Capacitance to do a quick check on stability

  • 0 in reply to Victor Salomon
    TI__Expert 7530 points
    Hi Victor,

    Yes, the input impedance ought to be in the 100's of MΩ range based on the input bias being in the single digit nA range. The MSP430FR2310 in the TSSOP16 package actually has a dedicated inverting input on the TIA to provide a very low bias current spec (5pA) for applications where this is critical. The dedicated pin was necessary to support this as muxing with digital IO functions adds leakage current in the other cases.

    I'll put in a request to see if we can provide some context around the differential input capacitance, but it may a week or two until I can get back to you on that.

    Regards,
    Walter

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