I am following this TI application note for PH meter design and I was advised by Sem from TI to use LM7711 chip in place of LM7721 (which was originally in the design note) for price consideration.
my circuit is on pcb , and i have kept the tracks short but I am getting strange behaviour from the circuit. Its putting out random voltages at the output and seem to loop between values from 0.0 to 50.0 .
I measured signals at two points and i have put the pics . PH1 shows the measurement at the junction of R1 and R2.
PH2 shows the signal at Vout.
why am i seeing an AC form instead of a DC
signal ? does using 7711 instead of 7721 requires some
circuit changes ?
Iam using the PH probe from a working PH meter so i know the probe is good.
please help me out with this since this is the only thing holding my project at
the moment.
I am also attaching the Eagle CAD circuit and board layout for this.
thanks