Re: horrible development

John Larkin wrote:

On Mon, 26 Feb 2007 02:05:44 GMT, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:


John Larkin wrote:


On Sun, 25 Feb 2007 20:04:59 -0500, Phil Hobbs
<pcdh@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:



John Larkin wrote:


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.

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.

Bummer.

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.

Any suggestions?


John



I've run into that too. Some parts connect all their outputs together when you exceed the supplies on one input. :-(


WHAT WERE THEY THINKING?



Could you raise the impedance of the voltage dividers a whole lot? Or replace one of the 4 resistors with a small cap and adjust the frequency of the test signal?


A couple of schottkies, clamping the low-ratio divider to +-5, seems
to work. Not too ugly a kluge.


The BAT54 comes in series pairs, making it one part. If it has to be super small even in SOT-523:

http://59.120.39.77/mccsemi/up_pdf/BAT54T,AT,CT,ST(SOT-523).PDF


See my discrete diode kluge in abse.


Nice solder job. Do I see a tin whisker south of TP57? Just kidding...


Actually, a sot-23 dual series schottky might be nice. If I solder pin
3 to the resistor junction, the ends run to +5 and -5 without crossing
over. I'll give that a try and see if it looks better.


The BAT54S comes in SOT-23:
http://www.nxp.com/acrobat_download/datasheets/BAT54_SERIES_4.pdf

About $0.03 in qties which is why I like them. Digikey has 72,000 of them in stock, which is kind of normal, which is why I like them even more. They'll still be around when I put my teeth in a jar. Imagine, a component that lasts a whole career.

--
Regards, Joerg

http://www.analogconsultants.com
.

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