Maker Pro
Login Join Maker Pro
Or sign in with

Menu

Connect With Us

Network

Maker Pro

horrible development

J

John Larkin

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
 
E

Eeyore

John said:
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?
Click to expand...

How fast are the signals ? I'd be inclined to actively buffer and clamp.

The other simple possibility is to fit your own ESD diodes in parallel and use
some shottkys or those very low Vf types.

Graham
 
P

Phil Hobbs

John said:
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
Click to expand...

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

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?

Cheers,

Phil Hobbs
 
M

Martin Riddle

John Larkin said:
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-----+--R5--| |
| | |----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
Click to expand...

Might be a easy solution, current limit the ESD protection. You got an OPamp
so Zin can be quite high.
See my edit in your circuit. R5

Cheers
 
J

John Larkin

How fast are the signals ? I'd be inclined to actively buffer and clamp.
Click to expand...

This (brain damaged) version of the module generates sinewaves from 0
to 100 KHz, but I run the self-tests at 10 KHz. Adding an active
buffer would be a frightful hack.
The other simple possibility is to fit your own ESD diodes in parallel and use
some shottkys or those very low Vf types.

Graham
Click to expand...


I found a couple of tubes of MAX4051's in stock, but they do the same
thing. How can people design analog mux'es that do this?

The fix is to add schottky clamps, the 1N5711's, from the 4:1 divider
output to the +-5 rails. It finally passes all the BIST routines. I
can go home now.

Too bad: this is rev A of this 8-channel DDS synthesizer board (pic
posted to abse a while back) and so far it has a single jumper;
somebody forgot to run the uP WRITE- signal to an FPGA. So now it's a
jumper and two diodes. Given a board this badly kluged, I suppose
there will have to be a rev B soon.

John
 
J

John Larkin

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

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?
Click to expand...

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

John
 
J

Joerg

John said:
See my discrete diode kluge in abse.
Click to expand...

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.
Click to expand...

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.
 
E

ehsjr

John said:
This (brain damaged) version of the module generates sinewaves from 0
to 100 KHz, but I run the self-tests at 10 KHz. Adding an active
buffer would be a frightful hack.





I found a couple of tubes of MAX4051's in stock, but they do the same
thing. How can people design analog mux'es that do this?

The fix is to add schottky clamps, the 1N5711's, from the 4:1 divider
output to the +-5 rails. It finally passes all the BIST routines. I
can go home now.

Too bad: this is rev A of this 8-channel DDS synthesizer board (pic
posted to abse a while back) and so far it has a single jumper;
somebody forgot to run the uP WRITE- signal to an FPGA. So now it's a
jumper and two diodes. Given a board this badly kluged, I suppose
there will have to be a rev B soon.

John
Click to expand...
Well, it may be a kludge, but wasn't it fortunate
you had room on the board? BTW C135 doesn't look like
it's a happy camper ...

Ed
 
F

Fred Bartoli

John Larkin a écrit :
This (brain damaged) version of the module generates sinewaves from 0
to 100 KHz, but I run the self-tests at 10 KHz. Adding an active
buffer would be a frightful hack.



I found a couple of tubes of MAX4051's in stock, but they do the same
thing. How can people design analog mux'es that do this?

The fix is to add schottky clamps, the 1N5711's, from the 4:1 divider
output to the +-5 rails. It finally passes all the BIST routines. I
can go home now.

Too bad: this is rev A of this 8-channel DDS synthesizer board (pic
posted to abse a while back) and so far it has a single jumper;
somebody forgot to run the uP WRITE- signal to an FPGA. So now it's a
jumper and two diodes. Given a board this badly kluged, I suppose
there will have to be a rev B soon.

John
Click to expand...

There are some switches designed to handle fault over voltage up to
(from memory) 44V.
I think they are marketed for automotive environments and ISTR that
there are some cloned on the 405x family.
The keyword is fault tolerant I think.

Don't have much time to search, but maxim comes to mind and probably
moto, oops onsemi.

If you go clamping and with a new rev. then I'd drop the leaky
schottkies, go with a BAV99, split the divider's top resistance and
clamp there.
 
J

Jan Panteltje

On a sunny day (Sun, 25 Feb 2007 16:00:32 -0800) it happened John Larkin
<[email protected]>:

You are using it out of spec, inputs should always be within clipping.
If it ever worked you were lucky!
Cannot you add a MOSFET or other switch to clamp the too high input to ground?
 
L

Lionel

WHAT WERE THEY THINKING?
Click to expand...

That designers would read the spec-sheet? It's not like there are lots
of chips that /don't/ dump over/under signal voltages to their common
rail.
 
J

Jim Thompson

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
Click to expand...

4052, or similar, so you can short the output leg of the 4:1 divider
to ground?

Or flip the 4051 backward so you pick a tap on a single divider...
(after all the pins are called I/O ;-)



...Jim Thompson
 
J

Jim Thompson

4052, or similar, so you can short the output leg of the 4:1 divider
to ground?

Or flip the 4051 backward so you pick a tap on a single divider...
(after all the pins are called I/O ;-)
Click to expand...

Ooops! Strike that second suggestion :-(

...Jim Thompson
 
J

John Larkin

That designers would read the spec-sheet? It's not like there are lots
of chips that /don't/ dump over/under signal voltages to their common
rail.
Click to expand...

The spec sheet names a maximum esd diode clamp current. It doesn't
mention that the chip becomes non-functional at about 1% of that
current.

John
 
J

John Larkin

On a sunny day (Sun, 25 Feb 2007 16:00:32 -0800) it happened John Larkin
<[email protected]>:

You are using it out of spec, inputs should always be within clipping.
Click to expand...

I think it is in spec, or at least that the spec is woefully
misleading. It's screwing up at about 1% of the epecified esd clamp
current.

John
 
J

John Larkin

Ooops! Strike that second suggestion :-(

...Jim Thompson
Click to expand...

Dang, you beat me to it.

But you can do a tapped divider and use the mux to ground taps on a
series-string divider, sort of like what you suggest.

But why do these chips do this? How does the esd diode current find
its way into the output? Incidentally, the effect is almost perfectly
symmetric for high and low clmping.

John
 
E

Eeyore

John said:
The spec sheet names a maximum esd diode clamp current. It doesn't
mention that the chip becomes non-functional at about 1% of that
current.
Click to expand...

We've discussed relaying on those diodes before a number of times.

It's the first time I've heard of it being problem but I'd not be inclined to
rely on them myself.

Graham
 
J

Jim Thompson

On Mon, 26 Feb 2007 07:19:19 -0800, John Larkin

[snip]
The spec sheet names a maximum esd diode clamp current. It doesn't
mention that the chip becomes non-functional at about 1% of that
current.

John
Click to expand...

I could be pompous and remind everyone how many times I've said DON'T
DO IT ;-)

...Jim Thompson
 
You must log in or register to reply here.

Similar threads

Audioguru
Horrible TV Audio and Video Lately
Replies
3
Views
395
(*steve*)
(*steve*)
D
Horrible 240V Wiring Job.
Replies
1
Views
1K
Metro
M
C
Horrible Hitachi - Last Call for Help
Replies
2
Views
875
John-Del
J
T
Horrible Harmonics!
Replies
2
Views
1K
Tony Williams
T
K
HP 1100 laser Printer -- prints horrible characters
Replies
10
Views
1K
Bob Shuman
B
Top