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AMC1301: Relation between Bias Current, R4/R5 and Gain

Kenichiro
Mastermind 9165 points
Part Number: AMC1301

[ AMC1301 ] Relation between Bias Current, R4/R5 and Gain

Hi,

Can you help me to understand the relation between bias current and Gain?

From the drawing and description in the datasheet, I understand that the bias current can be calculated with the equation below.

Vcm = (R4 + R5 ) * Ibias

Where
Vcm = 2V
R4 = 12.5kohm
Ibias = -60uA

So R5 = 20.8kohm.

WIth these estimation, the graph can be correctly obtained. Is this understanding correct?




Here, another point, I cannot understand is the gain.
The gain of AMC1301 is fixed 8.2. I would imagine that the combination of R4 and R5 shows the indication of this gain, however not...
As far as I check the datasheet of AMC1305, it seems that there is a relation between R4/R5 and gain.
Can you help me to understand this correctly?





Thanks,
Ken


  • 0
    TI__Guru**** 168485 points
    Hi Ken,

    You mention two different parts above - the AMC1301 (an isolation amplifier) and the AMC1305 (an isolated DS Modulator). Both devices have a similar front end circuit with gain = 4 (R4 = ~12.5k and R5 = ~50k). For the AMC1301, an additional gain factor of 2.05 is included in the back end.
  • 0 in reply to Tom Hendrick
    Mastermind 9165 points
    Hi Tom,

    Thank you for sharing information!
    I see that the gain(8.2) of AMC1301 is consisting of front-end gain (4) and back-end gain (2.05).

    Can you help me to understand the bias current calculation?
    Now I know my equation above is not valid (R5 = 50kohm, not 20.8kohm).
    How can I get the bias current value?

    Thanks,
    Ken
  • 0 in reply to Kenichiro
    TI__Guru**** 168485 points

    Hi Ken,

    The 60uA bias current is total current for both inputs to the AMC1301.  The current is measured with inputs shorted together and then tied to GND. It is ~30uA for each pin.

  • 0 in reply to Kenichiro
    TI__Mastermind 23050 points
    Hi Ken,

    If you would prefer not to use the plot of bias current vs. common-mode voltage, then alternatively you could approximate the bias current as ( Vcm - 2V)/(R4+R5), where the 2V is the common-mode voltage the output of the internal amp is biased to, Vcm is the input common-mode voltage, R4 = 12.5kOhm and R5 = 50kOhm. Multiply the result by two to get the total bias current on both pins. The result is very close to what we spec, but deviates slightly, which I suspect is due to the bias current of the internal amp.

    Regards,
    Zak Kaye
    Precision Amplifiers Applications
  • 0 in reply to Zak Kaye
    Mastermind 9165 points

    Hi, Tom, Zak,

    Thank you a lot for sharing the knowledge! This is what I wanted to know!
    I will consult with my customer!

    Regards,
    Ken