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Original question:

LMC6484: Output Resistance

LMC6484: Output Resistance

Kiran Kerai
Intellectual 380 points
Part Number: LMC6484
Other Parts Discussed in Thread: OPA320, OPA325, LMC6482

As mentioned in a previous thread:

In our application we have configured the opamp as a follower and have 270 ohm seried resistor connected to the output to a successive approximation ADC. As the opamp reaches saturation the voltage at the output of the opamp (measured by ADC) is non-linear (lagging).

We are using a MAX1230 ADC from Maxim that specified input resistance should be limited to <300 ohms and I suspect that the opamp output resistance is exceeding this resulting in non-linearity on measurement. <300 ohms is required because the ADC contains a capacitor on the input that must be charged during TRACK/HOLD cycle so that the input voltage is SAMPLED and measured correctly.

The LMC6484 opamp specifies approx. 110 ohms output resistance at Vs=5V. It was confirmed in previous thread that the effect we are seeing is a result of increase in output resistance as the opamp saturates.

We are now seeing this same effect but at lower input voltages, down to 4.4V. I would like to know:

What is the minimum voltage that we would should this behaviour?

Is there a different opamp that you could recommend to use to resolve this problem?

  • 0
    Intellectual 380 points

    I should mention that I have performed some testing which shows that the range 4.4V to 4.7V is affected by non-linearity. Op-amp supply rails are 5V.

  • 0 in reply to Kiran Kerai
    Guru 48175 points

    Well Kiran, 

    If you are running as a unity gain follower, you may be seeing the crossover network on the input side giving you non-linearity. 

    There are other parts that include an on chip charge pump to get RRin without a crossover network - this nice app notes shows some single channel options, 

    http://www.ti.com/lit/an/sboa181a/sboa181a.pdf

  • 0
    TI__Guru 61920 points

    Under linear condition the open-loop output impedance, Ro, gets devided by the loop-gain (AOL*beta) of the amplifier making close-loop impedance (Rout) look very small: Rout=Ro/(1+AOL*beta) - see below.  However, it is not devided by loop-gain when you allow the output to be trioded (in CMOS) or saturated (in bipolar transistor) because under such condition op amp's open-loop gain, AOL, collapses.

    Thus, in order to avoid distortion associated with the trioding of the output transistors, you must make sure that the op amp's actual output voltage swing, Vout, including voltage drop across the external resistor, is far enough from either rail - see below: 200mV to 650mV from rail depending on Iout and junction temperature. 

     

    No op amp's output may swing all the way to the rail but we have op amps that can swing much closer to rail than LMC6484.  Please take a look at OPA320 and OPA325, which can swing within less than 100mV of either rail and are best suited for driving ADCs.

    Please see attached for Ro vs Rout discussion:

    8372.Open vs Close Loop Zout.ppt

  • 0
    Guru 140200 points

    Hi Kiran,

    the LMC6482 is way too slow to settle to the final value within the aquisition time of your ADC:

    And you should avoid the supply rails for input and output signal. This will additionally slow down the OPAmp because the output must first leave the saturation (overload recovery time!):

    kiran_lmc6482.TSC

    Choose a much faster OPAmp which additionally specifies the settling time for settling to 0.01% of final value. Remember, you have a 12bit ADC! A specification of overload recovery time would also be nice.

    The OPA320 which Marek recommended is a much better choice:

    kiran_opa320.TSC

    Kai

  • 0 in reply to kai klaas69
    Intellectual 380 points

    Thank you for your response Kai and Marek!

    In our application the input voltage is DC. Does the settling time still apply?

    I agree there is some interaction with Rout and ADC sampling but I'm not sure a faster opamp will resolve the problem any more than an opamp with lower Rout.

  • 0 in reply to Kiran Kerai
    Guru 140200 points

    Hi Kiran,

    yes, even then the OPAmp must be able to settle quickly enough because the multiplexer and the tracking & hold process of ADC cause abrupt current changes:

    kiran_opa320_lmc6482.TSC

    Kai