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|3rd October 2013, 01:04 AM||#1|
Join Date: Oct 2011
Decompensated opamp oscillating? Fried opamps? PCM1794 IV Stage
I am implementing the PCM1794A IV stage from the datasheet with some ADA4627 and ADA4637 opamps.
The IV stage is the one found on page 20 of the datasheet.
NE5534 = ADA4627
LT1028 = ADA4637
I had a thread here about it. They measured quite well during simulation and yesterday I populated my PCB and it looks like i fried it by reversing +- rails ;(
So I plugged in my PSU and I see smoke come out of one of the caps or somewhere on the board and i quickly turned off the PSU.... I disconnected the PSU wires without noticing if I had them wrong and rewired making sure the + and - are correct... needless to say the outputs was at +11v DC constant... on both channels.
I think I fried my chips. Is it safe to assume that i fried all 6 ICs and that is why i get 11VDC at the outputs of the IV stage? ;(
So today I removed all the opamp chips and I only installed one ADA4637 chip and just powered up the board to measure the output.... expecting 0v but I had an AC oscillation of about 0.2Vpp and about 35Mhz signal (measuring on my 20MHz analogue scope).
Now the ADA4637 is decompensated and stable at gains 5 and above. Is that why I am getting this oscillation at the output?
Also the weird thing is that when I measure at the input IOUT+ on the datasheet page 20 I get this oscillating signal there as well! on the IOUT- i get nothing. Keep in mind that other ICs are not connected...
Can someone explain? Should I be worried?
This is not reality...
|9th October 2013, 04:40 PM||#2|
Join Date: Oct 2011
I recieved my new parts and have populated one of the channels and I got sound!
BUT there is noise on it... naturally :/
Ok so, I did fry the previous chips :/
About my setup:
- PCM2707 (USB) I2S -> SRC4192 -> PCM1794 -> I/V Stage for PCM1794
- Using an old computer power supply for all my voltages. +3.3V, +5V, +-12V
- The boards are stacked. 3 toal boards housing, USB, SRC+DAC, and I/V stage
- The output is fuzzy... high frequency fuzzy. The fuzz is about 20mVpp
- in the 1kHz signal test I have a "kink" in the sine wave and the 20mVpp fuzz
- when I touch the input of the L- (or was it L+?) I/V stage with my Scope Probe the kink goes away or moves to the very top and the sine wave sounds better!
- Music also sounds better withe the probe touching L-.
- Without the probe touching, there is some distortion in the music
- With the ADA4637 being the only chip on the board, the fuzz was there!
- The fuz is high frequency and my 20Mhz scope can barely see the peaks.
- the +-12 PSU has a 0.5v difference. +11.95V and -11.45V
- slight DC offset at the output (forgot to measure)
I found this Article very useful:
Op amps? G=1 stable & decompensated | EDN
I think it comfirms my theory about the ringing being generated from the decompensated ADA4637 due to "C3" feedback capacitor.
The DC offset at the output, due to the 0.5V difference in the +-12vDC PSU?
How about that kink in the 1kHz? That is waht causes my noise in music, the noise I can hear anyways. Probe on the current input, the kink goes away or hides to the top. Probe lifted from the current input, the kink come back. ??
I will try and get some screenshots tonight.... even though nobody has replied .
Also when I get sound without distortion to come out I will start a thread about the DAC as a whole and provide my schematics and BOM. It is not a unique implementation per se but I have spent a lot of time picking the right parts and in layout.
Anyhow, any feedback would be appreciated hehe
This is not reality...
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