Codeboy, that sounds as an very interesting project. Almost a vna like system. Very impressive results. I know how hard it is to get such a flat responce over that range. I have not tested CFB opamps. I use LT1028 and OPA37. The GBW is OK but I want to try opamps with a faster slewrate (I used good IC feed so it is not much work to change opamps in a later stage.(I need arounsd 10 of those so that is a bit expensive. I want to try a very fast LT opamp a friend tested, but those are around 8 euro a piece so I do not know yet is the extra investment makes it so more usable as it is now (I can measure power from DC to 500 kHz and 0V to 300V and 0- 10 A. RMS AC+DC , PF, phase, current, voltage, shunt temp, Watt and VA and I will use it 90% of the time to monitor 230V and current to the instruments I'm repairing or building)
Thanks. Yes, I wanted to have a different LCR meter, that wasn't just a few fixed frequencies, but I could set any test frequency from 20Hz to 1Mhz or sweep over the entire range and plot the output.
Yes, it's very hard to get the signal chain flat from 20Hz to 1Mhz, and in the end if I build it up on a PCB I might not be able to get it that good anyways; I will probably have to include some code to compensate; probably do a baseline sweep and use that to compensate for offset and gain errors in the signal chain. So maybe it's trying so hard to get it flat is not worth it after all and just a few dB down at the top and bottom end can always be compensated for in software, but I took a hardware approach and did my best to get it at flat as possible. I'm using two OPA727 and an LT6231 in a standard instrumentation amplifier configuration. I'm using the OPA727 (GBW=20Mhz, SR=30V/us) as gain=2 buffer, at the top and bottom of the DUT, followed by a LT6231 (GBW=215MHZ, SR=70V/us). There are two LT6231's, one is gain=2.5 (total gain=5) and the other is gain=2, for a total gain=10). I am probably going to have to add 2 more LT6231's in cascaded x2, x5 gains to get another gain x10, and reach a total gain of 100 (or a single LT6231-10 in gain=10, depending on price). The LT6231 has higher input offset voltages, but at the ends of the gain stage it is less and less of the error budget.
I like the high common-mode input impedance of the LT1028, it would make a nice buffer, but I don't think it's slew rate is high enough for signals larger than 1V @ 1Mhz.
Anything really good is $10, $15 or more.
Are you planning to make it sweeping ? A sawtooth X out to the scope and a two DAC's to make the Y signals. Or a small graphic lcd display (i'm learning C to couple analog design with digital, not to replace it. More like 1+1=3) My first project was a 2GHz counter and RF powermeter added as readout and powercontrol to an analog project I just finished (a 25MHz to 2000MHz sweeping signal generator)
Yes, I will make it settable test frequencies and sweeping across the range. I wasn't planning on scope output, but that sounds interesting.
Actually, I wanted to avoid a front panel at first, so I've just considered it to be USB controlled, and software on the PC. Then sweep it and graph the result on the PC like a VNA. I'll probably start like that.
In the powermeter I'm working on, I use a LTC2400 as ADC. Not needed it but I had a testboard in my junkbox for some time. That is the first time I use it but it works great (thanks to code I found and use as base and the SPI header)
Yes, I'm also considering whether I will use an off chip ADC like the LTC2400 too. This might improve the noise level as well, since I hear the horror stories of using a high-resolution ADC that's on the same substrate as the digitally switching micro... And with an off board ADC then I can use a (cheaper?) micro with a USB built in and no concern for it's ADC capabilities. As it stands, it's hard to find a reasonable cost micro with 16 or 24bit ADC and USB. It's easy to find a micro with USB and a 12-bit ADC, or a micro with no USB and a 16-bit or 24bit ADC. And I can always use one of the USB to Serial chips from FTDI, but I don't want to go that way if I can use a micro with built in USB.
Right now I'm at the early stages, and just seeing what performance I can get and where I can go with it.
That would be an impressive (almost impossible I think) result on a production unit. It is fairly doable on a one-off unit, with calibration.
Yes, I think so too. I can get it with my hand-tweaking on the bench, but I want to be able to specify say, a 2.2pf feedback bypass cap for gain limiting to reduce the peaking, and not be way off because the tolerance of the cap actually changes the gain peak so much that I get gains errors anyways (and thus can't read the voltage across the DUT accurately anymore).
So ultimately, I think all my effort to get a flat response like this might not be worth it, and in the end I'll have to run a calibration cycle anyways to get the baseline as built, and use that baseline to compensate and flatten the signal chain.