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LMP8603-Q1: Minimum Gain Setting

Jessica Riley
Intellectual 570 points

Part Number: LMP8603-Q1

Hello,

I would like to use the LMP8603-Q1 current sense amplifier in a system which has a variable current range (0~25mA, -1.5A~1.5A).  I would like to use the ability to reduce the gain by switching in the resistor from A1/A2 to ground.  For the lower current range 0~25mA, it will keep the resistor disconnected so the gain is 100.  For the higher current range -1.5A~1.5A, it will connect a 2.7kOhm resistor which should drop the gain to ~2.6 if my calculations are correct. 

Is there any issue using the current sense amplifier in this way?  

  • +1
    TI__Mastermind 20400 points

    Hello, 

    Thank you for your post. 

    Your approach looks good to me!

    I don't see any issues with what you're proposing. The LMP8603-Q1 is designed to handle gain reduction exactly like this, and your math checks out perfectly.

    Your gain calculation is spot on: Using the formula from the datasheet: Gr = (100 × Rr) / (Rr + 100kΩ) With your 2.7kΩ resistor: Gr = (100 × 2700) / (2700 + 100000) = 2.63 V/V ✓

    Why this works well: The chip's internal design actually makes this pretty straightforward. They bring out the signal path between the preamp (10x gain) and output buffer (10x gain) on pins A1 and A2 specifically so you can do things like this. When you connect that resistor from A1/A2 to ground, you're just creating a voltage divider with the internal 100kΩ resistor.

    A few things worth noting:

    • This gain reduction method is stable - the instability warnings in the datasheet only apply when you're trying to increase gain using positive feedback
    • The device handles output saturation and configuration changes just fine (TI engineers have confirmed this on other E2E threads)
    • You should be all set for bidirectional ±1.5A sensing with OFFSET tied to VS/2

    Just a couple practical things to keep in mind:

    • Whatever switch you use (analog switch, MOSFET, relay, etc.) should have low on-resistance compared to 2.7kΩ, and minimal leakage when open
    • Double-check that both current ranges keep your output voltage within the linear range (roughly GND+0.3V to VCC-0.3V)
    • Give it a moment to settle after switching between gain modes

    What type of switching approach are you planning to use? And what's your shunt resistor value and supply voltage? That might help me think of any other considerations.

    Best Regards, 

    Joe