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Part Number: THS4551
Hello everyone, I'm writing this because I've tried to understand why some things were happening when simulating a design and I've not reached a definite conclusion on the problem. My best guess is something to do with the THS4551 specifications that I'm probably not seeing.
What I'm trying to accomplish is to amplify and filter a sound signal coming from an electret microphone as seen in the attached image. This signal is AC cuopled and then feed to the differential amplifier as a single-ended input, in order to apply gain and filter the frequencies I'm not interested in. The voltage generated by the mic is around 50-100 mV peak to peak.
The bode curve that results from this setup does what I need, however, calculating the gain as Rf / Rg = 100, which would be 40dB, the bode doesn't quite get there (it's approx 26db).
If I try to increase the feedback resistance Rf (or decrease Rg) and simulate the transient response of a squared-wave of 50Hz and A=50mV the circuit turns unstable (I think).
Note: The limit on the top and bottom are caused by the zener diode, this is on purpose to avoid any overvoltage going through and damaging the ADC. I'm fairly certain this shouldn't affect the performance but I added them just in case.
The output will be directed to a multiplexer and then an ADC, both differential.
One additional thing is that I simulated the Open-Loop Gain and there's approximately 80° of Phase Margin at around 800kHz.
So finally I don't know if I'm missing something but I guess asking wouldn't hurt.
If you need any more information I'll gladly comment further.
your circuit is very unusual. Why do you use a 150MHz differential amplifier to amply a 50Hz signal from a electret microphone? Why not using a simple OPAmp for this?
The 1W zener diode 1N3825 is short-circuiting the output of THS4551. This conflicts with the built-in short-circuit protection circuitry of THS4551. You need to increase R3 and R4 (R1 and R2 in my simulation):
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In reply to kai klaas69:
In reply to Santiago Anzorena:
In reply to Michael Steffes:
then why not using the recommended scheme in figure 48 of datasheet of ADC161S626?
And you can even omit the VCM offsetting scheme consisting of the two 1µ caps and 10k resistors at the input of ADC161S626, if the output signal of your THS4551 is already centered arround a suited 2.5V common mode voltage.
Looks acceptable. Eventually, add 4.7p caps across Rf1 and Rf2 to improve the noise margin. As the feedback resistances are unusually high for the THS4551 and are far away from the recommended 1k (see table 5 of datasheet), still carry out a phase stability analysis though.
Man, I have seen that picture so many times I don't know how I didn't think about that.
Truly amazing, I have followed your changes and performed a quick phase margin calculation and it is exactly 45º.
Now the problem is that the 0db-cross is close to where all the phase shift happens related to the poles of the amplifier, so a slight variation in the value of the capacitors parallel to Rf1 and Rf2 would mean a big shift in phase.
From a few tests, I get the picture than I can't use any cap above 4.7p since that would be working within marginal stability, and the frequency of the pole is around 72kHz. Now if I use a 2.2pF cap, that frequency becomes 153kHz but on the plus side the phase margin is 60º approx. I could even stretch further and use a 1pF cap to get an 88º phase margin, with frequency 338kHz.
Simulation with C = 4.7pF
Simulation with C = 2.2pF
I may need some insight wether it's fine to just attenuate everything above 338kHz or I need to compromise a bit of margin and get a better attenuation in lower freqs, thus improving noise performance I assume?
here you see my phase stability analysis of your circuit:
The phase margin is 84° which is pretty much perfect. And it turns out that adding 4.7p caps across the feedback resistors is no good idea.
See also this:
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