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horrible development


John Larkin

Jan 1, 1970
Perhaps using a different device would be a more optimal solution,
particularly for the higher voltages experienced.

Where I worked previously, we designed an automatic test jig that
utilised NAIS AQW212A PhotoMOS devices, and they worked very well.
That series had devices that worked up to 600V @ 40mA load current.
The last price I obtained from Mouser (early 2006) was roughly US$2 each
for the '212A's.


This board already has 48 (!) SSRs in the channel switching paths, not
to mention 9 real relays for the BIST switching. But a 6:1 mux with
ssr's would take more room and more port pins than I have.

We're using a Claire (alternate NEC) part that's about 85 cents in
moderate quantity.

I agree, ssr's are very handy, even in situations where "relay" is an
improbable choice. Think of them as isolated-base transistors.


Rich Grise

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 analog mux:

| |
| |
| |
bus--->------+-------r1-----+------| mux |-->--opamp-->--adc
| | | hc4051 |
| r2 | |
| | | |
| gnd | |---- +5
| | |
| | |---- -5
+-------r3-----+------| |
| | |----gnd
r4 | |
| | |
gnd ________

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.

I've been kind of following this thread, and I have a question: did I miss
the part that said, "Well, yeah, the 21:1 divider protects the input "for
the big stuff"" without noticing that the input of the 21:1 divider and
the input of the 4:1 divider both follow the same signal?

IOW, what have you got protecting the 4:1 input from the 25Vpp that
results from dividing 100V by 4?


James T. White

Jan 1, 1970
Richard Henry said:
My most recent bug (alas, undetected at peer review):

It was a PIC-based USB-interfaced display controller. I was doing the
hardware design and most of the software, with one complicated routine
provided by a software writer who had done similar functions on other

I worked through most of the Holiday shutdown (with the promise of a
makeup week off in April, when I had a week-long family event
planned), over the next two weekends, and late Monday night, finally
getting a display in the shipping box just in time for the last FedEx
drop to our customer (who was having a demo for _his_ customer the
next day). The hardware worked ok, but the software running on the
test PC occasionally got the answer wrong for some of the data coming
over the USB link.

The next morning, I looked over the complete PIC listing trying to
find the problem, and found out that the routine written by the
software guy that I had plugged in to my code was sending out the USB
message it had encoded at the end of the routine. Unfortunately, I
was also sending out the message right after the routine returned. I
went running off to my boss to tell him I had a fix, only to find out
that since we had succesfully made the shipment the night before, most
of the team was being laid off. Right now. Pack your office.

I'll never make _that_ mistale again.


Hopefully your boss delivered his news before you delivered yours.