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'[PIC] Measuring Analog Voltages w/o Sharing Ground'
1999\10\28@102613 by Andy Kunz

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I'm working on a data logger which is a little different than what I've
done before.

I have three independent circuits (Power, Control, and Monitoring).
Control and Monitoring _may_ share a ground, but this is neither necessary
nor preferred by the customer.

Power must be as isolated from Control or Monitoring as possible, as the
Power circuit gets a lot of noise on it and Control is susceptible to noise :-(

My project is to provide data to the Monitoring system (and ground and
power may both be shared).

Required data:

a) Average Voltage to Power circuit.  Power is a 6V-24V source to a 70A
load switched at 3KHz to control motor speed.  The motor has a nasty back
EMF spike.  We need to see the average voltage during the last 100mS.
b) Average Current in Power circuit.  Average current during the last 100mS.
c) Voltage of Control circuit.  Control is a 3-6V source for a 75mA-300mA
typical load, with occasional 1A loads.  The power supply is well able to
provide 1A with minimal drop for short periods.
d) Voltage of Monitor circuit.  Monitor is a 4.5-5.5V source (I'm
investigating whether they will use NiCd or Li technology).  It is a PIC
driving an Atmel serial DataFlash chip (which I'm not sure yet, probably
the 4M piece), so it will have bursts of "high" current.  Supply will have
no problem providing this.

All data points will be sampled at 10-30 Hz (selected by customer at
runtime, with no hardware changes permitted).  8-bit data is sufficient at
this stage, but they may want 10-12 bit in the future.

The software (PIC) is essentially done - I just borrowed another data
logger I did.  The hardware is a problem.

The Monitor circuit is supposed to be small (as small as possible) and
needs to run at least 1 hour on batteries (NiCd rechargeable or Li coin
cells).  Of course it's mobile, and will use an RS-232 interface to
transfer data to a laptop, either during operation or at the end of a
testing period.

Test points for the Control circuit is an existing jack into the system
(power and ground are on the connector, not a problem).

Test points for the Power circuit are another matter.  A connection to the
positive side of the supply is very easy, and to the negative side almost
as easy.  Connection for the ammeter function would be easily accomplished
on either the high side or the low side of the supply, but it needs to
introduce as little resistance as possible (they don't want to hurt
performance of the system).

Now my question:

How do I hook these separate analog signals up without sharing the ground
with the Power circuit?

Thanks.

Andy



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1999\10\28@123249 by Dwayne Reid

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>I'm working on a data logger which is a little different than what I've
>done before.

<snip>

>
>Now my question:
>
>How do I hook these separate analog signals up without sharing the ground
>with the Power circuit?
>

It sounds as if your customer has SOME budget for moderately expensive
parts.  I'd use a 2 port or 3 port isolation amplifier.  I've used both the
Burr-Brown 3656 and the Analog Devices AD210 - I prefer the AD part.  They
cost about $50 USD and are worth every penny in situations such as yours.
They require a single 15 Vdc power source - this source can be isolated from
both the input and output.  The input looks like a standard op-amp - you
treat it as such when setting gain, etc.  The output is a unity gain op-amp.
There is auxiliary power available at both the input and output side of the
module.  The AD part is good for 10 or 12 bits of accuracy (I don't remember
the exact linearity spec).

PS - the difference between a 2 port and 3 port amplifier: the 2 port
generally shares a common ground between the output and power supply; the 3
port is completely isolated between input, output and power supply.  The
AD210 is spec'd at 2500 VAC between input and both other ports; 1000 VAC
between power and output.  They are tested and must pass at MUCH higher
voltages than those.

There are other ways around your problem, but the easy ones (much harder
than the isolation amplifier) don't have much accuracy.

Hope this helps.

dwayne


Dwayne Reid   <dwaynerspamKILLspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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1999\10\28@125706 by Maverick

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>a) Average Voltage to Power circuit.  6V-24V
>b) Average Current in Power circuit.  Average current during the last 100mS.
>c) Voltage of Control circuit.  Control is a 3-6V source
>d) Voltage of Monitor circuit.  Monitor is a 4.5-5.5V source

>All data points will be sampled at 10-30 Hz (selected by customer at
>runtime, with no hardware changes permitted).  8-bit data is sufficient at
>this stage, but they may want 10-12 bit in the future.

>The Monitor circuit is supposed to be small (as small as possible) and
>needs to run at least 1 hour on batteries (NiCd rechargeable or Li coin
>cells).  Of course it's mobile, and will use an RS-232 interface to
>transfer data to a laptop, either during operation or at the end of a
>testing period.

>How do I hook these separate analog signals up without sharing the ground
>with the Power circuit?

Depends on what your design constraints are, obviously - how much additional
cost is acceptable, etc...  Do you intend to use the internal PIC A/D
converters or
an external part?  If you are using an external A/D, is that set in stone
or flexible?

This post intregued me because I've spent the last four years working
on EV telemetry and power systems, and we often face similar problems in
terms of noise,
coupling, floating voltages, and you name it...

Personally, I'd recommend you use true differential delta-sigma A/D
converters - and no,
before everyone jumps on me, these aren't necessarily out of the price
range on any reasonable
project - Analog's AD7705 is a relatively inexpensive (~$4 in small
quantity, I think) dual differential 16 bit D-S A/D that has an
SPI interface and works very nicely under extremely noisy conditions.
Because of the
integration-deintegration performed in the D-S process, noise and random
jitter in the data is reduced
significantly - to the point that one of these was used on the output of a
large (500W) switching power
supply to measure current through a shunt- and had consistent and accurate
average currents out to 10 to 12 bits
without a problem.  And with a main ground bus that can have ~2-3 volts of
ripple on it due to currents and inductive
kickback, this is no small feat!   Power consumption on these is very
reasonable for battery-based applications - I believe
they can be standby'd to 10uA or so...

If that isn't an option or you would prefer to use some other A/D
converter, just put an instrumentation amp on the
front end - these usually have two drawbacks:  they're expensive if you go
for really high-quality versions, and otherwise
their common mode rejection isn't the greatest at higher frequencies.
Since you're only looking for averages, though, you
could R-C integrate off all the high frequencies and leave the in-amp to
only convert the differential input signal you're
trying to monitor down to a "ground" referenced voltage for your circuit to
read.  I'm personally a fan of Burr Brown INA2128s, but
these are costly if you actually have to use them in a production circuit.
Analog also makes some very nice ones at lower cost, I
believe.  For your application, they also have one other drawback I just
thought of - they're usually current hogs.  On the order of ~20mA.


Just my two bits for the day...

ND Holmes




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1999\10\28@130104 by Maverick

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Concerning my earlier post concerning instrumentation amps or differential
ADCs, I feel I should mention
after reading Dwayne's post I rethought my assumption that these signals
are all loosely referenced to some theoretical "ground", your
problem was just that there was excessive noise floating around on the
power ground for the circuit that you didn't want
messing up your readings or coupling back into the signals being sensed...

If that isn't the case, pretty much everything I said is meaningless drivel
and just ignore it...

ND Holmes
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1999\10\28@131749 by Andy Kunz

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>If that isn't the case, pretty much everything I said is meaningless drivel
>and just ignore it...

But you said what I wanted to hear - don't take it all back!!!

I think Dwayne interpreted me more correctly.  I'm looking to keep these
other systems isolated from the Power system.  Not just because of noise,
but because it's possible that they are connected inadvertently and I don't
need to let magic smoke out of somebody else's equipment, let alone mine,
by picking up "ground" in different places.

What I need is some way to measure the Power parameters without being
electrically connected, in a way of thinking.  I would opto-isolate if
necessary, etc, but that gets into other problems (like real estate).

Andy

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1999\10\28@132553 by Jim Hartmann

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A simple technique requiring a capacitor and DPDT relay:

Vx+ >----o | o-----> AtoD
          |
         +-)(-+
             C |
          +----+
          |
Vx- >----o | o-----> GND

Relay on to sample Vx, relay off to measure.

1999\10\28@150342 by Quentin

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First cross post, hehe

Quentin

1999\10\28@160030 by Andy Kunz

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At 12:22 PM 10/28/1999 -0500, you wrote:
>A simple technique requiring a capacitor and DPDT relay:
>
>Vx+ >----o | o-----> AtoD
>           |
>          +-)(-+
>              C |
>           +----+
>           |
>Vx- >----o | o-----> GND

I need to make up to 30 samples/sec.  That's a pretty nasty buzz for a relay.

I like the concept, though.

Other ideas?

Thanks.

Andy
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1999\10\28@163501 by Don Hyde

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I was thinking along the lines of using another PIC (like maybe 12C671)
powered from the motor supply making the motor measurements, but
electrically isolated from everything else, then sending a bit stream
through an optocoupler to the PIC with the EEPROM.  It's cheap (<$2 in 25's
from Digikey), tiny (8-pin), and has an 8-bit A/D.

> {Original Message removed}

1999\10\28@164548 by Andy Kunz

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At 03:29 PM 10/28/1999 -0500, you wrote:
>I was thinking along the lines of using another PIC (like maybe 12C671)
>powered from the motor supply making the motor measurements, but
>electrically isolated from everything else, then sending a bit stream
>through an optocoupler to the PIC with the EEPROM.  It's cheap (<$2 in 25's

Well, DUH!!! Why didn't I think of that.

You forgot one point - I HAVE THEM IN STOCK!!!!  How Stupid can I be!

Thanks.

Andy

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1999\10\29@064151 by fernteix

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Hi

Use the same  idea with analogue swiches.

fernando

-----Original Message-----
From: Andy Kunz <EraseMEsupportspamMONTANADESIGN.COM>
To: RemoveMEPICLISTEraseMEspamEraseMEMITVMA.MIT.EDU <RemoveMEPICLISTspam_OUTspamKILLspamMITVMA.MIT.EDU>
Date: Quinta-feira, 28 de Outubro de 1999 21:00
Subject: Re: [PIC] Measuring Analog Voltages w/o Sharing Grounds


{Quote hidden}

relay.
{Quote hidden}

1999\10\29@205426 by Harold M Hallikainen

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       This is a "flying capacitor" analog isolator.  As I recall, Maxim makes
a chip that does this.

Harold


On Fri, 29 Oct 1999 11:28:36 +0100 fernteix <RemoveMEfernteixKILLspamspamTELEWEB.PT> writes:
> Hi
>
> Use the same  idea with analogue swiches.
>
> fernando
>
> {Original Message removed}


'[PIC] Measuring Analog Voltages w/o Sharing Ground'
1999\11\01@141811 by Erik Reikes
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At 10:31 AM 10/28/99 -0600, you wrote:
>>I'm working on a data logger which is a little different than what I've
>>done before.
>
><snip>
>
>>
>>Now my question:
>>
>>How do I hook these separate analog signals up without sharing the ground
>>with the Power circuit?
>>

This is similar to a problem I was just working on.  We were measuring
current loops, so we could power the isolated side of the circuit.  I'm not
sure if that's an option for you here.  What we did is basically just power
an AD off of the current loop via a zener and send the serial signals
through optoisolators.

Some of the other things we thought about are :
1: analog optoisolators ....  really crappy temp coefficient kills your
accuracy over temp.
2: Voltage to frequency using op-amps then through opto isolators or caps.
Uses a fair amount of power on the iso side.  This problem can be solved by
having a truly isolated supply on the other side using a transformer.
3: A sort of charge sharing setup where storage caps are switched from iso
circuit to internal circuit.  I never quite got this one, but the EE's tell
me it should work.
4: Voltage to PWM through opto-isolators.  Also somewhat of a power hog and
sort of complicated.

HTH

-Erik Reikes

1999\11\02@133011 by Andy Kunz

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At 07:02 AM 10/29/1999 -0700, you wrote:
>        This is a "flying capacitor" analog isolator.  As I recall, Maxim
makes
>a chip that does this.

The MAX619 is the only chip with "flying capacitor" anywhere, according to
their search engine.  Got another name for it?

Thanks.

Andy

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1999\11\02@140733 by William Bross

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Andy,

If you need a 'flying cap' front end, you may want to take a look at the
LTC1043 Dual Precision Instrumentation Switched-Capacitor Building Block
from Linear Tech at http://www.linear-tech.com/

Bill

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