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LM7171 Testing

Salil Kamra
Prodigy 110 points
Other Parts Discussed in Thread: LM7171

Hi,

I am using LM7171 in non inverting amplifier configuration. Tina schematic of the circuit is below:

The results in TINA show output as pulses varying between 13.65V and -13.65V with rise time of some 39nsec giving slew rate of about 558V/us.

Here are the actual results that have same component values with same supply range.

Here, it is seen that LM7171 is taking 1.4us to swing between -13.2 and 13.6 V, hence giving slew rate of about 14V/us.

Why is this considerable difference is there. Also, when datasheet says 4100V/us, why in TINA even it gives 558V/us.

Look forward for reply.

  • 0
    TI__Genius 13935 points
    Hello Salil,

    Are you sure the LM7171 is the limiting factor in your lab setup?
    What is the bandwidth of your oscilloscope? What kind of probe are you using?
    What is the rise time for the signal generator?

    Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    TI__Genius 13935 points
    Hello Salil,

    What is the purpose of the 39pF capacitor in the feedback loop?
    The circuit is open loop for DC. That runs the risk of saturating the outputs. There should be at least an inductor in parallel with the RC feedback. An appropriately valued resistor would work as well.

    Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    TI__Mastermind 22825 points

    Hi Salil and Loren,

    I think Loren is right about not having DC feedback path. You are probably "railing" the output in both directions and you could be seeing overload recovery from output saturation. The swing values you quoted (~ +/-13.65V) is another indication that you are railing the output since output swing should be less than +/-13V. This would not be the way I would look for output slew rate.

    Looking at LM7171 Figure 29 (Large Signal Pulse Response, Av= +2, Vs=+/-15V) plot, you see the following:

    SR = ~ 20 V / 4ns = ~5,000 V/us

    Also, it is best to zoom in on the rising / falling edges to make sure the scope "measurement" is not being fooled by having a slow time-base.

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Prodigy 110 points

    Hi Loren and Hooman,

    Thanks for taking out your time to reply.

    This op-amp is used as an error amplifier in a circuit for constant current load. The output of the opamp is given, in my application, to the gate of a MOSFET, which is expected to act as gate to source voltage for FET, and hence determine the current through it.

    The capacitor 39pF is used as a compensation element to test the circuit for fast rising transients, as without this capacitor what I could analyse is , there would be plenty of ringing in the circuit operation.

    Above is my complete circuit. Maximum voltage that my application permits on the non inverting terminal is 0.1V, so as to sink 30A from this 0.9 source.

    However, during testing:

    a) the resistor with value 500 ohm was not there

    b)the output of the opamp was isolated from mosfet.

    c)Non inverting terminal was given pulses ranging between 0-3.3V while voltage at inverting input was given 2V.

    So ideally, the circuit was expected to work as comparator, during testing, which it did, but with very less slew rate.

    Again going through datasheet, I could figure out that slew rate limitation is due to the input differential voltage.

    When I increased the amplitude of the pulses to 10 V at non Inverting , while reducing to 1V at inverting, the rise time decreased significantly to ~250ns for same swing at the output.

    Still, the slew rate was less.

  • 0 in reply to Salil Kamra
    TI__Mastermind 22825 points

    Hi Salil,
    As you've also noted, the LM7171 slew rate is a strong function of input differential voltage as shown in Figure 23 (Slew Rate vs. Input Voltage) where it ranges from 600 V/us (x-axis = 1Vpp), to just over 3,000 V/us (x-axis = 10Vpp). With closed loop operation, large differential input voltage occurs during transient while the output is trying to catch-up to where it should be. Therefore, the slew rate you achieve is also dependent on how fast your input can slew (I cannot tell from your explanation if you have monitored / measured that?).

    You are essentially operating the device open-loop and you need a large, and fast differential voltage to get the fastest slew rate you can get because you don't have the normal feedback path that delivers a jolt of differential voltage during transience to increase output slew rate. Another thing to try is to make sure the output is never railed (you may be able to experiment with diodes to fixed voltages which limit the output to +/-12V or less).

    Regards,
    Hooman