On a board, we have several identical channels of an analog signal generation chain. A single channel consists of an AD5452, followed by ADA4807-4. We use all 4 opamps on the ADA4807 package to construct a 4th order filter at a cutoff between 150-200khz. Our DAC's are driven by an FPGA.
AD5452 is in bipolar mode. It's reference voltage is 1.5V. AD4807 is driven by 1.5V and -1.5V.
In order to test the board, we generate a sine-wave around 60khz frequency from all channels. Our SPI speed is 50MHz. For a period of a sine-wave, we generate 32 samples only.
When we test the board, we see that *some* of these channels have a peculiar noise on the output, especially towards the negative part of the sine wave. Attached, you can see the waveform shape. If we zoom in to the bottom peak of the sine-wave, the "noise" frequency is approximatly 50MHz
Schematics of a single channel is below:
Also, we look at the output of the 3rd opamp in the above schematic, (to the left of R4), and the signal appears to be very clean at that point.
Our supplies appear to be clean as well. We tried adding a small capacitor (~100pf) at the output of the last op-amp, but it made the noise worse.
Any ideas what might be going wrong here?
A quad is not four op amps in one package, it is four functions in one piece of silicon. At high
frequencies, you have coupling with stray C to substrate, through the bond wires, between
the traces on the pc board, etc. If you have capacitance on the inverting input of an op amp,
you form a pole that leds to instability. See:
Planet Analog - Articles - Op amps: to dual or not to dual? (Part 1 of 2)
Planet Analog - Articles - Op amps: to dual or not to dual? (Part 2 of 2)
I emailed the filter wizard author for comments, but he is out this week.
With respect to real estate problems, If you make a tiny board, but it doesn't work, then what?
"we did not remove all the metal...." you will have to do a board spin before you go into production.
You could also try a slower op amp: ADA4891-4, AD8618, AD8648, or ADA4084-4.