A-D front end - robust and for high voltage

[Gorilla Nerfball]

Can anyone shed some light on how multimeters, scopes and other
similar devices can measure anything from microvolts to 300V or more
without any moving parts (relays, etc.). Specifically, how do I design
the front end for an A-D capable of measuring up to 100V without
sacrificing performance at lower signal levels too much?

What about safety considerations? How do I isolate a 120V input from
the user, especially when that same front end has to measure
millivolts.

My application is a home brewed, flexible data logging/low freq
oscilloscope device. I want to be able to handle reasonably high
voltages (120V if possible), but at the same time be able to measure
millivolt waveforms via a high gain ins-amp at the front end. What if
I limit myself to something like 30V input, does that simplify things?

In the end, I need a high impedance input that's robust and can switch
between mV measurements and V measurements without any moving parts
(i.e. relays).

Any thoughts on where to start?

Thanks,

Chris

 

What is "too much"? There are a lot of games that you can play with
attenuators (switching the low side, and relying to some extent on
input protection / fixed input limiting resistor) that fall apart
when
you go from essentially DC to a much wider bandwidth.

I think I'd like to get 8 bits of meaningful resolution at 1mV P-P, and
at least 12, ideally 16 bits at higher signal levels (10V P-P).
Bandwidth wouldn't need to be too high, ideally 100kS/s I'll be happy
with 10kS/s.

I understand what you mean about the input protection. If I was limited
to only a couple of volts, I'd just switch gain on an instrumentation
amp, letting it clip to its heart's content if the signal got too high.
But at 100V, it'd fry the whole thing.

What about safety considerations? How do I isolate a 120V input from
the user, especially when that same front end has to measure
millivolts.

Input resistor and low-leakage input protection.

I get the input resistor, the other thing I'm not familiar with. Assume
for a second that I have little or no experience with the practical
side of this type of electronics. Can you shed some more light on what
you mean by low-leakage input protection, perhaps in terms of a
circuit?

My application is a home brewed, flexible data logging/low freq
oscilloscope device. I want to be able to handle reasonably high
voltages (120V if possible), but at the same time be able to measure
millivolt waveforms via a high gain ins-amp at the front end. What
if
I limit myself to something like 30V input, does that simplify
things?

What's the highest resolution that you will need? Can you attenuate
the input, and have variable gain thereafter?

16bits at 10V P-P, maybe 8 meaningful bits (after figuring noise, etc)
at 1mV P-P. If I attenuate the input, doesn't that inherently mean that
I attenuate my, already small, 1mV signal too?

In the end, I need a high impedance input that's robust and can
switch
between mV measurements and V measurements without any moving parts
(i.e. relays).

Bandwidth? Resolution/accuracy for smallest inputs?

Again, 100kS/s ideally, 10kS/s would be ok.

There are some really cool optically coupled semiconductor "relays"
(NAIS) but most of these have somewhat large leakage and capacitance
for these applications. The last I knew 'scope makers were still
using
relays {I would be happy to hear if this information is out of
date,
especially if that included how it was done...}
Thanks for you help,

Chris

 

[Apostrophe Police]

On Thu, 27 Jan 2005 20:05:33 -0500, Mark Jones wrote:

Hmm. I bought an older ('93) Iwatsu DS6121 DSO from military surplus.
It works well and has a series of small relays inside that switch the
active preamp. They barely even make a "click" sound. (It also has two
slots full of "74F" series logic chips comprising the RAM - could
^^^^^^^^^^
probably save a couple hundred watts if they were changed to CMOS!)

An Apostrophe Police Medal of Honor, to the one who is the first I've
seen use that word properly in over 25 years.
--
Rich Grise, Self-Appointed Chief,
Apostrophe Police
;-)
[0] Yes, that's not a sentence. Ain't that a bitch! ;-)

 

[Keith Williams]

In article <pan.2005.01.28.04.38.58.244029@111.111.111.111>,
sendspamhere@111.111.111.111 says...

On Thu, 27 Jan 2005 20:05:33 -0500, Mark Jones wrote:

Hmm. I bought an older ('93) Iwatsu DS6121 DSO from military surplus.
It works well and has a series of small relays inside that switch the
active preamp. They barely even make a "click" sound. (It also has two
slots full of "74F" series logic chips comprising the RAM - could
^^^^^^^^^^
probably save a couple hundred watts if they were changed to CMOS!)

An Apostrophe Police Medal of Honor, to the one who is the first I've
seen use that word properly in over 25 years.

Your kidding.

--
Keith

 

[John Larkin]

On 27 Jan 2005 09:51:28 -0800, gorilla_nerfball@hotmail.com (Gorilla
Nerfball) wrote:

Can anyone shed some light on how multimeters, scopes and other
similar devices can measure anything from microvolts to 300V or more
without any moving parts (relays, etc.). Specifically, how do I design
the front end for an A-D capable of measuring up to 100V without
sacrificing performance at lower signal levels too much?

DVMs have big manually-operated rotary switches, as do analog scopes.
Most digital scopes do in fact have relays; you can hear them click as
you change vertical ranges.

John

 

[Tom Del Rosso]

"Mark Jones" <abuse@127.0.0.1> wrote in message
news:6d2dnSb3SdRAEmTcRVn-uQ@buckeye-express.com...

Now you would run all these taps to something like a 4066 digital
switch (provided one of these can handle at least +15v in the "on"
condition, but they may not like +155v in the "off" condition - please
check its datasheet. A series of opto-isolators and transistor
"switches" might work in place of the 4066.)

I think this part of the issue is the part that the OP was asking about, and
I don't think any of the respondents really addressed it. Obviously a 4066
would fry.

My Fluke meter has no relays (and damn few of any type of components). It
does have what looks like a resistor network on a 1cm x 4cm substrate with
several taps, but nothing that looks like a solid state relay. There are
several SMT transistors, but only one or two TO92s.

--

Reply in group, but if emailing add
2 more zeros and remove the obvious.

 

[kmaryan@gmail.com]

Perhaps it's time for me to spend some money on a multimeter and a good
pry bar...

Thanks for all the posts, they've given me a bit to work on. In
general, I think I'm going to end up making special gain stages
separate from the main DAQ. Connectors are cheap and it won't take much
to snap on a high gain or high attenuation module to an otherwise
ordinary +/-10V DAQ front end.

Chris

 

[Tom Del Rosso]

"John Larkin" <jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote in
message news:2cusv0t5f4lpir7js33ai530hlj7ebc1vp@4ax.com...

DVMs have big manually-operated rotary switches, as do analog scopes.
Most digital scopes do in fact have relays; you can hear them click as
you change vertical ranges.

But how do autoranging DVMs do it with only a few SMT transistors?

--

Reply in group, but if emailing add
2 more zeros and remove the obvious.

 

[John Larkin]

On Tue, 01 Feb 2005 15:27:13 GMT, "Tom Del Rosso"
<ng01@att.net.invalid> wrote:

"John Larkin" <jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote in
message news:2cusv0t5f4lpir7js33ai530hlj7ebc1vp@4ax.com...

DVMs have big manually-operated rotary switches, as do analog scopes.
Most digital scopes do in fact have relays; you can hear them click as
you change vertical ranges.

But how do autoranging DVMs do it with only a few SMT transistors?

Big resistors. That's easy for DC or low-frequency AC. Scopes are
wideband and can't tolerate large unbypassed resistors, so need
switched dividers. Solid-state switches still have too much
capacitance and can't tolerate overvoltages, so most scopes still use
a relay or two in the sront-end.

John

 

[keith]

On Tue, 01 Feb 2005 19:11:35 +0000, Tom Del Rosso wrote:

"John Larkin" <jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote in
message news:q6avv05kt8pnhoiv7k39a5932cfdk5a3qj@4ax.com...

Big resistors. That's easy for DC or low-frequency AC. Scopes are
wideband and can't tolerate large unbypassed resistors, so need
switched dividers. Solid-state switches still have too much
capacitance and can't tolerate overvoltages, so most scopes still use
a relay or two in the sront-end.

Doesn't the "big resistor" divider have to be switched too?

Sure, but "big resistor" isn't so sensitive to "small switch resistance"
and "low bandwith" doesn't have to have the high capacitance of the
FET switches compensated for. It's all about bandwidth.

--
Keith

 

[Mark Jones]

keith wrote:

On Wed, 02 Feb 2005 13:30:51 -0500, Spehro Pefhany wrote:


On Wed, 2 Feb 2005 12:59:28 -0500, the renowned Keith Williams
krw@att.bizzzz> wrote:


In article <Qt_Ld.134653$w62.104419@bgtnsc05-
news.ops.worldnet.att.net>, ng01@att.net.invalid says...

"keith" <krw@att.bizzzz> wrote in message
newsan.2005.02.02.02.54.40.96579@att.bizzzz...

Sure, but "big resistor" isn't so sensitive to "small switch resistance"
and "low bandwith" doesn't have to have the high capacitance of the
FET switches compensated for. It's all about bandwidth.

I'm sorry I haven't been very explicit about what has me baffled. What I
don't understand is how those small switches can take the high voltage
sometimes present. For low ranges I think there has to be a switch near the
top of the divider, right? If so, why doesn't it get fried when there's a
high-voltage input?

I don't believe the switches on a DMM are on the input side, rather in
t he feedback of an instrumentation amp. It would hard to maintain a
multi-megohm input resistance with a resistor divider hanging on.

Instrumentation amp? In a DMM?


Perhaps not. I was thinking about how to get the Vin up there, but not
hard enoguh, apparently. ;-)

Someone who is more knowledgeable (has ripped one apart could better
comment here. You're right though the few hundred volt input tollerance
is impressive.

Something like this I think: -

o-----[10M]----------x--------x--------x----------x-----> LPF & chip
| | | |
[1.11M] [101K] [10K] [low leakage clamp ]
| | | |
o o o o | +/-400mV in
400mV / \ 40V 400V |
4V | |
o |
| |
0--------------------x----------------------------x------> LPF & chip


The switch never sees more than a few volts or a few tens of uA.


So the switch is then make before break? ...or does one assume the clamp
will work?

I think that's the best idea yet. Perhaps replace the switch entirely with a
tri-state device? (assuming the parasitics aren't going to munge the rest of the
circuit.)

The uC has the job of keeping the divider at the highest range, until a signal
change is detected, then it ramps down until the reading is within range.
Perhaps it can detect high voltages without conduction by electrostatic means?
(A FET gate running parallel to the input trace --> latch --> timer?) No, that
wouldn't work when in the highest range and a small voltage were applied. Maybe
then there is a high-impedance frequency generator imposed on the divider, and
the amplitude delta triggers ranging? Or for that matter, what is going to be
able to measure 400mV at Z=10MOhms in the 400v scale? We need a pro's answer.


-- "Sometimes it is best to just shut up while you are ahead." MCJ 20050119

 

[Spehro Pefhany]

On Thu, 03 Feb 2005 09:25:47 -0500, the renowned Mark Jones
<abuse@127.0.0.1> wrote:

Or for that matter, what is going to be
able to measure 400mV at Z=10MOhms in the 400v scale? We need a pro's answer.

Straight into the meter chip. Ib ~= 1pA typical. That includes the
clamp diodes.

In autoranging meter ASICs, they would use "double pole" switches on
the die because the "on" resistance of CMOS switches is rather high.
They would also use differential input to the meter circuitry
(switching the current path separately from the measurement path--
basically a Kelvin connection). An ADC with differential inputs (or an
instrumentation amplifier plus a single-ended ADC) could be used in an
non-ASIC version.



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com