Matched diodes

[Raveninghorde]

I want to clamp a 10mV signal to 0V.

A matched common anode diode would do it with 1 diode fed from a
positive supply so that the anode was a diode drop above 0V and the
other diode clamping to this point. Easy in principle.

I looked at some specs such as BAT54A, and various other parts but so
far I haven't found a part which specifies the matching of Vf. Any
suggestions?

 

[Jamie]

I want to clamp a 10mV signal to 0V.

A matched common anode diode would do it with 1 diode fed from a
positive supply so that the anode was a diode drop above 0V and the
other diode clamping to this point. Easy in principle.

I looked at some specs such as BAT54A, and various other parts but so
far I haven't found a part which specifies the matching of Vf. Any
suggestions?

You don't say if this 10mv is sub level of a larger DC, nor do you
say if it's DC or AC ?

So for the moment, I'll assume it's a 10mv DC base line that you want
to squelch out

Pass the 10mv into something like an op-amp first stage pre amp.

A gain of 10 would then bring this signal to 100mv. The output
would then be a reference to use on a comparator which can be used
to switch something on/off..

A comparator, being that the output on most that i've seen are
open collector, will let you easily integrate this into a analog
switch.. For example, a T bar Diode switch can be used where the
comparator output (via a R load) would supply the center bias for the
switch via


Freq response time, max input and things like that has to be known..

P.S.

On the preamp stage, you wrap the loop back gain with a zener to
avoid over drive on the input.

It's seems to be a trivial problem, but maybe there are some
constraints you are not mentioning here?


Jamie.

 

[Raveninghorde]

You don't say if this 10mv is sub level of a larger DC, nor do you
say if it's DC or AC ?

So for the moment, I'll assume it's a 10mv DC base line that you want
to squelch out

Pass the 10mv into something like an op-amp first stage pre amp.

A gain of 10 would then bring this signal to 100mv. The output
would then be a reference to use on a comparator which can be used
to switch something on/off..

A comparator, being that the output on most that i've seen are
open collector, will let you easily integrate this into a analog
switch.. For example, a T bar Diode switch can be used where the
comparator output (via a R load) would supply the center bias for the
switch via


Freq response time, max input and things like that has to be known..

P.S.

On the preamp stage, you wrap the loop back gain with a zener to
avoid over drive on the input.

It's seems to be a trivial problem, but maybe there are some
constraints you are not mentioning here?


Jamie.

Thanks folks for the responses.

It's a dc input, which has to be measured down to 10mV, to an MCU
successive approximation AtoD. However the signal can go negative by a
diode drop, say -0.6V. The input has a schottky diode holding it to
-0.2V.

However the MCU gets the measurments wrong when the input goes to
-0.2V. I don't know the mechanism and at the moment I don't know if it
is this input or another input that is screwing up.

I do know the MCU makes the wrong decisions. If the input is clamped
to 0V when at -0.2V everything works OK. The board is in China so I
was looking for a cludge they could try to prove the point.

I didn't think matched diodes would work but it was worth asking the
question. John gave me an answer that told me to look at other
solutions. I've gone for a 2 transistor circuit driving a FET to
clamp the pin when the input goes negative, and removed the schottky.

I'll know by breakfast whether that has worked.

I am curious how -0.2V can screw the Samsung MCU AtoD. It's not enough
to forward bias the catch diodes but if it got into the substrate it
could change the voltage range. I wondered if the comparator in the
AtoD was doing a phase reversal due to the negative input but it seems
unlikely.

It's an interesting challange sorting out the Chinese, it's a shame it
has to be done under time pressure.

 

[Raveninghorde]

Source impedance?

...Jim Thompson

100 ohm input resistor from a low impedance point. I've asked that to
be changed to 10k.

 

[Spehro Pefhany]

Thanks folks for the responses.

It's a dc input, which has to be measured down to 10mV, to an MCU
successive approximation AtoD. However the signal can go negative by a
diode drop, say -0.6V. The input has a schottky diode holding it to
-0.2V.

However the MCU gets the measurments wrong when the input goes to
-0.2V. I don't know the mechanism and at the moment I don't know if it
is this input or another input that is screwing up.

I do know the MCU makes the wrong decisions. If the input is clamped
to 0V when at -0.2V everything works OK. The board is in China so I
was looking for a cludge they could try to prove the point.

I didn't think matched diodes would work but it was worth asking the
question. John gave me an answer that told me to look at other
solutions. I've gone for a 2 transistor circuit driving a FET to
clamp the pin when the input goes negative, and removed the schottky.

I'll know by breakfast whether that has worked.

I am curious how -0.2V can screw the Samsung MCU AtoD. It's not enough
to forward bias the catch diodes but if it got into the substrate it
could change the voltage range. I wondered if the comparator in the
AtoD was doing a phase reversal due to the negative input but it seems
unlikely.

I suppose it's possible that it's outside the common mode range of the
comparator, but that seems unlikely.

Are you sinking a lot of current with the micro? Has it been
confirmed by measurement that the input is actually at -200mV or are
you surmising that it "must be"?



Best regards,
Spehro Pefhany

 

[Jamie]

Thanks folks for the responses.

It's a dc input, which has to be measured down to 10mV, to an MCU
successive approximation AtoD. However the signal can go negative by a
diode drop, say -0.6V. The input has a schottky diode holding it to
-0.2V.

However the MCU gets the measurments wrong when the input goes to
-0.2V. I don't know the mechanism and at the moment I don't know if it
is this input or another input that is screwing up.

I do know the MCU makes the wrong decisions. If the input is clamped
to 0V when at -0.2V everything works OK. The board is in China so I
was looking for a cludge they could try to prove the point.

I didn't think matched diodes would work but it was worth asking the
question. John gave me an answer that told me to look at other
solutions. I've gone for a 2 transistor circuit driving a FET to
clamp the pin when the input goes negative, and removed the schottky.

I'll know by breakfast whether that has worked.

I am curious how -0.2V can screw the Samsung MCU AtoD. It's not enough
to forward bias the catch diodes but if it got into the substrate it
could change the voltage range. I wondered if the comparator in the
AtoD was doing a phase reversal due to the negative input but it seems
unlikely.

It's an interesting challange sorting out the Chinese, it's a shame it
has to be done under time pressure.

You need to elevate your ADC input with an offset!

It's common practice to do this, You will of course
loose some dynamic range when doing this. If you bring the
input up to lets say .5 volts, and in the software you account
for this offset, you'll be fine.

Jamie

 

[John - KD5YI]

You could use an open collector comparator to clamp the input to
ground when pulled below zero.

...Jim Thompson

Right. In other words, an active clamp.

John

 

[Raveninghorde]

It worked.

I suppose it's possible that it's outside the common mode range of the
comparator, but that seems unlikely.

Are you sinking a lot of current with the micro? Has it been
confirmed by measurement that the input is actually at -200mV or are
you surmising that it "must be"?



Best regards,
Spehro Pefhany

The micro spec says 2 Meg input impedance.

The actual measurement was - 190mV.

 

[Raveninghorde]

On Thu, 06 Jan 2011 00:04:27 +0000, Raveninghorde

SNIP

I do know the MCU makes the wrong decisions. If the input is clamped
to 0V when at -0.2V everything works OK. The board is in China so I
was looking for a cludge they could try to prove the point.

I didn't think matched diodes would work but it was worth asking the
question. John gave me an answer that told me to look at other
solutions. I've gone for a 2 transistor circuit driving a FET to
clamp the pin when the input goes negative, and removed the schottky.

I'll know by breakfast whether that has worked.

I am curious how -0.2V can screw the Samsung MCU AtoD. It's not enough
to forward bias the catch diodes but if it got into the substrate it
could change the voltage range. I wondered if the comparator in the
AtoD was doing a phase reversal due to the negative input but it seems
unlikely.

The MCU AtoD gets a positive reading when the input is -0.2V. The
adjacent AtoD channel is unaffected. So a negative input voltage
screws the successive approximation conversion.

 

[Spehro Pefhany]

It worked.


The micro spec says 2 Meg input impedance.

Sure, I meant on other pins.. outputs.. if there is enough current
flowing then you could have more than 200mV to the internal level of
Vss.

The actual measurement was - 190mV.

Curious.


Best regards,
Spehro Pefhany

 

[PeterD]

I want to clamp a 10mV signal to 0V.

Why diodes, just use a piece of wire, that will clamp to 0 volts every
time.

Troll...

 

[Raveninghorde]

Sure, I meant on other pins.. outputs.. if there is enough current
flowing then you could have more than 200mV to the internal level of
Vss.

Curious.


Best regards,
Spehro Pefhany

1 output feeding 2 100k resistors to transistor bases.