Hi,
I have a couple of questions regarding using LM675 as improved Howland current source to drive an inductive load. The circuit is shown below. R7, L1 and C1 represent the equivalent model of the inductor when measured with a LCR meter. The problem I am seeing is that the circuit is not stable. Without any signal input the output would oscillate between positive and negative rails at around 850 Hz. my questions are as below,
1. In LM675 datasheet Figure 15 the RC snubber is connected directly at the op amp output while in my case it is in parallel with the load. which location is a better one? http://www.ti.com/lit/ds/snosbp3e/snosbp3e.pdf
2. In TINA I couldn't find the spice model for LM675. Can someone provide a model for this IC? alternatively, can someone advise which IC might have similar performance and can be used instead of LM675?
3. I tried to measure the frequency response with output as the voltage across R9 (VR9 = Iout/2). I see only amplitude peaks at 2 times at around 1KHz. the phase shift is much less than 180 deg. the circuit should not oscillate according to the classic negative feedback theory. however, the below circuit has both positive and negative feedbacks. I am not very sure how to analyse it.
4. I am going to try increasing C2 to stabilize the circuit. any comments on this?
5. the inductor assembly in the application might vary from batch to batch. If parameters vary too much the circuit might become not stable again. If I want to set a LCR specification on this inductor how should I spec it? To spec the resonant frequency to be within certain range?
Thanks in advance
BR
Gordon