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LMP93601 - Input Bias Current

John Schimandle
Intellectual 390 points
Other Parts Discussed in Thread: LMP93601

On page 5 of the LMP93601 data sheet, the input bias current is specified as -1.3nA. I'm seeing about 90nA bias current through the 2.2M resistor connected to the VCM pin of the LMP93601. I have nothing else connected to the IN+/- input except for the 2.2M resistor. I see 1.1V on the VCM side which is withing spec for the 3.3V supply voltage. On the IN+/- side I see 0.9V which is about 200mV of voltage drop across the 2.2M resistor. This calculates out to about 90nA of input bias current.

This design uses two LMP93601 parts and both parts have the same issue on all 6 inputs (IN+ and IN-).

Is this expected?

  • 0
    Intellectual 390 points
    SPI is working as I can write and then read back register values. The register values I have written are the same ones I've read back. So it looks like the digital interface is working.

    XCLK, ADC sampling and conversion are working or at least it looks like it is. I can stimulate SYNC with a positive edge aligned with XCLK as defined in the data sheet. Then I get a negative edge on DRDY at the appropriate time. I can read back the ADC values. I was expecting to see values very close to zero but I'm not seeing that. That's what prompted me to look at the analog inputs on IN+/-.

    The configuration register write order is as follows,

    LMP93601_WRITE | LMP93601_REG_LOCK | LMP93601_REG_LOCK_UNLOCK,
    LMP93601_WRITE | LMP93601_REG_CONFIG1 | LMP93601_REG_CONFIG1_SINGLESHOT | LMP93601_REG_CONFIG1_CH1_2_3,
    LMP93601_WRITE | LMP93601_REG_CONFIG2 | LMP93601_REG_CONFIG2_VCMENABLED | LMP93601_REG_CONFIG2_SPS_1057,
    LMP93601_WRITE | LMP93601_REG_CONFIG3 | LMP93601_REG_CONFIG3_DIGITALGAIN_32 | LMP93601_REG_CONFIG3_ANALOGGAIN_64,
    LMP93601_WRITE | LMP93601_REG_CONFIG4 | LMP93601_REG_CONFIG4_CONVERSIONMODE,
    LMP93601_WRITE | LMP93601_REG_ALARMMASK,
    LMP93601_WRITE | LMP93601_REG_SDO_CFG | LMP93601_REG_SDO_CFG_SDOHIGHZ,
    LMP93601_WRITE | LMP93601_REG_LOCK | LMP93601_REG_LOCK_LOCK,

    I can send you all the define values if you like but I think you get the idea. Unlock the device, write the configuration, lock the device. Then start sending SYNC pulse properly aligned with XCLK. Digital gain is set to 32 and analog gain is set to 64.

    The device is basically set up for 3 channel, round robin sampling and conversion. Gains are fixed for all inputs and not changed. Then a SYNC pulse is generated every 1mS. The device asserts ~DRDY about 140 uS prior to the next SYNC pulse. The ADC values for all three channels are read in the 140 uS window prior to the next SYNC pulse arriving.

    Supply voltage is 3.3V. Separate LDOs for AVDD and IOVDD.

    I can provide schematics and the current set of modifications if you need them.
  • 0 in reply to John Schimandle
    TI__Genius 16890 points
    Hi John,

    I need a little more detail on the schematic. Do you have both resistors (R1, R2) installed as shown in Figure 30?

    Thanks,

    Mike
  • 0 in reply to Michael Stout
    Intellectual 390 points
    Mike,

    Yes. Both R1 and R2 are installed on all three inputs to the LMP93601. A total of 6 resistors for each LMP93601.

    John
  • 0 in reply to John Schimandle
    TI__Genius 16890 points
    Hi John,

    Could you please send me the schematic and also information on the sensor that you are using?

    My email is mike.stout@ti.com

    Mike
  • 0 in reply to Michael Stout
    Intellectual 390 points

    Bottom line: This is pilot error on the measurement. Thanks to Mike for pointing that out. I will close this one out.

    Let me summarize:

    The scope I was using has a 10M input impedance and this created a voltage divider with the 2.2M pull up resistor. That's why there is a 200mV drop.

    I came up with an alternate measurement idea. So instead of measuring the voltage at IN+, IN- with respect to GND, I can measure the voltage drop across the 2.2M resistor with my high input impedance DVM. This should provide a measurement without disturbing the system.

     

    Actual measurements with a battery operated DVM EXTECH MA220 are 5mV drop across the 2.2M resistor. The DVM has an input impedance of 7.8M. So it looks like the current is about 3 nA which is much closer to the specification. I tried my more accurate DVM which is AC powered and I got a different reading depending on the polarity of the leads so I figured there was some sort of path to GND through the DVM. That’s why I switched to the battery powered DVM. The battery powered DVM has consistent results regardless of polarity.