I’m building a power meter using the AD8307 log amplifier. The circuit itself is almost as simple as the one they show in the datasheet (figure 32 page 17, http://www.analog.com/static/importe…ets/AD8307.pdf), the only difference is that I added 100K resistors to ground near C1&C2 (between the caps & IC) to eliminate charging of the caps from internal bias currents. I also added a 0.1uF cap from pin4 to ground and a R-C filter tuned at about 100 Hz near the output jack. The layout is kept pretty minimal with the input being a small loop as possible after an isolation transformer.
What I am trying to measure is a multi-channel RF amplifier which outputs signal at 480KHz with AM. The amplifier is connected to the power grid through an isolation transformer that prevents leakages to the ground at the output freq. The outputs (2 in this case) are hooked up to a 30dBm attenuator which goes to an Agilent power meter (for verification = gold standard) and runs in series to my power meters. My AD8307 meters are housed in septate aluminium boxes (Hammond 1590B style) and are fed from isolated power supplies. Internally I use a DC-DC converter which offers 1500V isolation. After the DC-DC converter there is a 10uF for ripple filtering before going to the circuit as shown below.
The amplifiers are outputting about 20W max and there is only a few % crosstalk seen on the Agilent power meter. However on my power meters I can see as much as 100% crosstalk. The output from the AD8307 goes to an NIDAQ card which can sample both in single ended and differential mode, however when hooking up to ground I see noise.
The input jack is an insulated BNC which goes into a 1:1 isolation transformer which goes to feed the circuit. The output jack is a normal metal case BNC which is shorted to the case of the meter to offer grounding (if I am sampling in single ended mode).
Everything is hooked up using BNC cables so shielding is pretty good. Does anyone have any suggestions about where to look for crosstalk source? I checked the AD8307 meters and they work very well on their own, however not when two are hooked up.
Thanks!
- Please note that the resistors after the input caps are in fact 100K and that there is an 0.1uF cap from pin 4 to ground not shown in the schematic.
