LT6203 Stability Analysis

Question

I am trying to evaluate the stability of an LT6203 driving a capacitive load in LTspice, but I am not sure if my AC loop-gain testbench is correct. 
 I attached my current LTspice schematic and AC plot. In this setup, I tried to break the feedback loop and inject an AC source, then plot V(in_m)/V(fb). However, the result does not look meaningful. I suspect my biasing or loop-breaking method is wrong. 
 I am also not sure how to interpret the datasheet for this load. The datasheet shows overshoot vs capacitive load, but the plots only go up to 1000 pF, while my load is 2 nF with a 20 ohm series resistor. Therefore I cannot tell from the datasheet whether this load should be expected to cause excessive overshoot, ringing, stability problems, or any issue related to driving a high capacitive load. 
 
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ianguieb · Accepted Answer

Hi trebole, Before evaluating the AC loop gain, it is important to first make sure that the circuit has a valid DC operating point. If the loop is opened in a way that removes the dc feedback path, the amplifier input/output bias can shift or saturate, and the result will not be meaningful. Here is a sample simulation of the open loop gain: I have attached the LTspice file here: 1121.LT6203 Open loop gain.asc 
 Regarding the capacitive load, the datasheet plots are useful as a guideline. A 2 nF capacitive load is relatively heavy for this type of amplifier, so stability should be verified by simulation and, ideally, transient response testing. The 20 ohm series resistor will help isolate the capacitive load from the amplifier output, but the final behavior depends on the circuit configuration, feedback network, output swing, and layout parasitics. 
 Best Regards, Ian)

SLA Status	Assignee	Support Status
SLA Closed. Met after 63 hours	MStokowski	Awaiting Customer Response

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