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AMC1351-Q1: AMC1351-Q1 combined with Voltage-Follower

Aaron_Rangel
Intellectual 450 points
Part Number: AMC1351-Q1
Other Parts Discussed in Thread: AMC1351

Hello,

I'm using this isolated amplifier AMC1351-Q1 to monitor a voltage busbar (1200Vdc max).

I have included a voltage follower between the divisor resistors and the input pin of this isolator amplifier.

See attached capture to check details of the connection.

The intention to do this was to add independency on the impedance between the divisor resistors and the isolated amplifier.

After checking the isolator amplifier datasheet, I wanted to know whether this is redundant and I can get rid off this stage or it can be used without affecting the input impedance of the isolator amplfier (Zinput of isolator amplifier in parallel with Zout of Voltage Follower).

As to input capacitor, in case of use it, does it make sense to move it to the resistor RSNS as it is indicated in the attached capture?

Regards,

  • 0
    TI__Guru 56532 points

    Hi Aaron,

    I don't think this voltage follower will be necessary. 

    The AMC1351-Q1 has an input impedance of ~1.25Mohm, which will be in parallel with the sensing resistor. We also have the AMC1311B-Q1, 0-2V input with a 1Gohm impedance. Depending on your resistor divider design and what kind of calibration will be performed will determine which device will provide highest accuracy. I recommend taking a look at this excel calculator to quantify the error of both devices to help you decide: https://www.ti.com/lit/zip/sbar013

    If you do elect to keep the voltage follower, I think moving the capacitor close to the resistor divider is the correct choice as this is where the majority of the noise will come from and additional resistance would be necessary on the output of the voltage follower to keep the input the AMC1351-Q1 stable. 

    Please let me know if you have additional questions. 

  • 0 in reply to Alexander Smith
    Intellectual 450 points

    Hi Alexander,

    Ok, I get you about the impact on the impedance and seemed to me that it was going in that direction.

    Maybe I can remove this voltage follower and at the end it is a matter of accuracy to select the appropiate device depending on the requirements.

    I will check this amazing calculator you have recently sent.

    Another point I wanted to ask you is about the fact of adding a zener diode in the input pin of the isolation amplifier to clamp the voltage in case of unexpected event in the busbar.

    With the current setup I've sent before, voltage follower included, would it work? See attached capture.

    I mean, adding zener diode in parallel with RSNS = 3,79kOmh could change the voltage measured (normally internal resistance of zeners are a few of Ohms). Let's say I add a zener diode BZT52C5V1-7-F.

    On the other hand, adding it in the input pin of the isolation amplifier could change the input impedance.

    In case of removing the voltage follower (I think I'm for this one) adding a zener in the input pin of the isolation amplifier would impact on impedance, too.

    So, to sum up, is it addequate to connect a zener diode on any of the before mentioned cases?

    Regards,

  • 0 in reply to Aaron_Rangel
    TI__Guru 56532 points

    Hi Aaron,

    We typically recommend to not add Zener's to the input pins as it can create some undesired complications, like the ones you've mentioned - but also other problems during EMC type testing. 

    For the AMC1351, the input voltage can go up to 15V before risk of permanent damage. Furthermore, it is the current specification in the table below, 10mA that is more concerning. If you are concerned about damage the device then I recommend sizing the resistor divider in a way that prevents more than 10mA from flowing to the input pin, where less current is preferred. 

    By the way, what type of application is this for? 

    Is there a sales person from TI that you are in contact with? 

  • 0 in reply to Alexander Smith
    Intellectual 450 points

    Hello Alexander,

    My final decission is to remove this Zener diode after checking the voltage that this pin can withstand and confirmation about all related to adding this component.

    I have 5x180kOhm resistors in series connected to resistor Rsns = 3,79kOhm, Vdc = 1200V max, so I'm not going to manage currents higher than 2mA in this leg.

    Therefore, we will be far away from the limits in terms of current.

    Voltage follower will be removed, too.

    Thank you very much for your feddback!

    Really appreciate it.

     

    Regards,