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Original question:

LMP8603-Q1: Question about datasheet

LMP8603-Q1: Question about datasheet - page17

John Li2
Prodigy 50 points
Part Number: LMP8603-Q1
Other Parts Discussed in Thread: LMP8602, LMP8601, LMP8603

Thank you Javier for your reply and your advice on the dependency of the input impedance wrt the Vcm. Much appreciated.

One more question is about the figures (Figure49-Figure52) shown on page17.

Although it says OFFSET pin is connected to Vs for these measurements, these figures indicate that the OFFSET pin is actually connected to ground - Vout is close to zero when Vin is negative, Vout vs Vin appear to be linear when the Vin is positive, and when Vin=0, Vout=0 (it would have been 2.5V if OFFSET were connected to 5V)

Also these measurements seem to apply only to LMP8601 which has a gain of 20 rather than LMP8602 and LMP8603.

Do I understand it correctly?

Kind Regards,

John

  • 0
    TI__Mastermind 26485 points

    John,

    Again you are correct.  I will also note that for those for the datasheet update.  I will need to go through this datasheet more thoroughly and I will give you any updates as I see them or fix them. 

    If I see any information I think is very important I will let you know. 

    Please give me the part variation(gain option) you plan on using and any specific information on how you plan on using this device.  I will need a few of days to dig deeper into this datasheet and look for other errors and see if I need to forward you any other information.

  • 0 in reply to Javier Contreras4
    Prodigy 50 points

    Thank you Javier for confirming this

    My application is fairly simple and straightforward - at least I hope so...

    I plan to use the LMP8603 (Gain-100) to monitor a current flowing from the device to an external negative power supply. The negative power supply is generated from a programmable DC/DC regulator which supply a voltage range from -3.0V to -3.65V. As the maximum current consumption of the device is 10mA, I plan to use a 3.7ohm resistor between pin8 and pin1 with Vs=5V. The max. voltage drop across the resistor would be 37mV, and the voltage on the device would be 37mV higher than the negative power supply, just above 1.2% in the worst case.

    So the maximum input voltage Vin=3.7ohm*10mA=37mV, with unidirectional setting (OFFSET pin connected to GND) the output voltage of LM8603 is expected to be 3.7V. This leaves a margin of 5V-3.7V=1.3V which corresponds to a headroom of 1300mV/(3.7*100)=3.5mA.

    Assuming the measurement accuracy of my equipment for Vout is +-5mV , the current measurement accuracy would be around +-(5mV/(3.7ohm*100) = +-14uA.

    The current is flowing from the device to the negative supply, so pin8 (Vin+) is connected to the device pin while pin1(Vin-) is connected to the negative power supply with the resistor 3.7ohm being connected between pin8 and pin1.

    As the CMV can be down to -20V, I think this simple design would work...

    Kind Regards,

    John

    BTW, if you guys are looking for some engineer based in UK home,  please do not hesitate to contact me :-)

  • 0 in reply to Javier Contreras4
    Prodigy 50 points

    Hi Javier,

    Attached please find the schematics I created, please could you advise me if anything I missed in the design, Thank you.

    Kind Regards,

    John

  • 0 in reply to John Li2
    TI__Mastermind 26485 points

    John,

    Everything looks good.   

    Some notes:

    Schematic has two notes on VBIAS_0.  We already covered the limited CM range and you would only use 5V Supply for CM below -4V. 

    If you do populate C4 your capacitor will be limited and most amplifiers cannot drive a large capacitive load.  You could place a resistor in series similar to that of figure 59 in the datasheet.  The issue is you need to move the location so that your Rf1 is effective to increase your gain.

  • 0 in reply to Javier Contreras4
    Prodigy 50 points

    Great point, Javier. Thank you.

    Kind Regards, John