- Jan 1, 1970
OK, we have a BIST (built-in self-test) bus that can have sine waves
from a couple volts to about 100 volts p-p. So I did this into an
bus--->------+-------r1-----+------| mux |-->--opamp-->--adc
| | | hc4051 |
| r2 | |
| | | |
| gnd | |---- +5
| | |
| | |---- -5
| | |----gnd
r4 | |
| | |
where one divider is about 4:1, for low level signals, and the other
is about 21:1 for the big stuff. ADC range is +-3.5, and I can take a
lot of samples and average to get dc, and simultaneously average the
abs value to get ac.
What happens is that when I have a big signal, selecting the
high-ratio divider, the output of the 4:1 divider blows through the
esd diodes of the mux and sneaks its way into the output, so I get a
lot more signal than the 21:1 attenuated level I want, and it's of
course distorted as well.
Bummer. One of the HC designers once assured me this wouldn't happen,
but that was another vendor (Moto) and the parts we have here are
Fairchild so I guess different processes can do this.
So, I can kluge on a couple of 1N5711's as clamps, really ugly, or
find another drop-in part that doesn't blow through.
I haven't looked at all the posts with regards to this problem, so please
forgive me if I'm reposting someone else's answer.
Since the bus is common to both inputs, except with the attenuation,
you will be getting 25volts at the 4:1 input when the bus is at 100volts.
This vastly exceeds the maximum input voltage of this device when you're
only supplying it with +/- 5V DC.
Increase the attenuation of the 4:1 input to 10:1, and you shouldn't have
any more problems.
No wonder you're damaging the input protection diodes and getting
unwanted feedthrough. Got nothing to do with the chip manufacturing