I am breadboarding an amplifier/LP filter circuit using an INA125 instrumentation amp, ECS 1 MHz clock to generate signal for the filter cutoff, a NAND gate to boost the clock output (thanks to Don Bruder for explaining this in an earlier post), and a Maxim 281 Bessel LP filter. The amp is powered at +- 15 V (using the breadboard's power supply), and I am using LM7805 and 7905 voltage regulators to provide +- 5 V for the rest of the chips.
I breadboarded the amp, clock, and NAND gate successfully, with all components working properly. The trouble started after I added the Maxim chip.
At this point I have checked and rechecked my connections, but the following two problems remain: 1) the output of the NAND is saturated at
+5 V rather than generating a square wave, and 2) the output of the Maxim filter is malformed. It is sinusoidal (like what I'm putting in) but it flatlines every time it crosses ground. The LP filter is working, however, with the right cutoff set.Like this, but with curvature to the sinusoidal part. (12 point Courier)
- * * * *
It does this with other waveforms, too.
I can input the clock signal directly into the LP filter, skipping the NAND, and the right cutoff is still set, but the output is still malformed. (Maybe I don't need the NAND, but since I'm eventually going to serve several circuits with the same clock, I'll keep it for now.)
I have separated my analog and digital grounds (connected in one place only). Another thought I had was that since I am using a quad NAND gate, do I need to do anything to the other three gates? There are only 4 pin connections currently: 1A (input), 1B (grounded), 1Y (output), and ground.
I feel like it's an analog/digital problem, but I don't know how to isolate it since I have to have the clock chip running to use the Maxim. The thing I can think of to do next is to take the Maxim and its connections off the board to see if the NAND output corrects, and make sure it didn't get damaged by something I may have done. I don't want to just switch in another one in case my board is currently a Toxic NAND Gate Killer.
Also I suspect the voltage regulators, but they do seem to be functioning correctly.
I would be very grateful for any help.
Amanda