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Input impedance on an instrumentation amp
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spam.zachr@gmail.com
electronics forum beginner


Joined: 11 Jul 2006
Posts: 4

PostPosted: Tue Jul 11, 2006 4:56 am    Post subject: Input impedance on an instrumentation amp Reply with quote

I must be missing something obvious... I am working on the front end
of a differential ADC system that is designed to measure battery cell
voltages. I am using an 4052 feeding a AD623. I really need a high
input impedance >1meg. According to the data sheet, when measuring
"floating" input sources (the batteries are definately floating), you
must provide for a bias current path. The recommended circuit uses two
100k resistors from each input to ground. That would make the input
impedance ~200k or less. That's way to low for my application. What am
I overlooking?

Thanks,

Zach
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Rene Tschaggelar
electronics forum Guru


Joined: 24 Mar 2005
Posts: 540

PostPosted: Tue Jul 11, 2006 5:31 am    Post subject: Re: Input impedance on an instrumentation amp Reply with quote

spam.zachr@gmail.com wrote:
Quote:
I must be missing something obvious... I am working on the front end
of a differential ADC system that is designed to measure battery cell
voltages. I am using an 4052 feeding a AD623. I really need a high
input impedance >1meg. According to the data sheet, when measuring
"floating" input sources (the batteries are definately floating), you
must provide for a bias current path. The recommended circuit uses two
100k resistors from each input to ground. That would make the input
impedance ~200k or less. That's way to low for my application. What am
I overlooking?

How about biasing one input to say 1V with a
resisitve divider and having the other open ?

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
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Eeyore
electronics forum Guru


Joined: 22 Jun 2006
Posts: 642

PostPosted: Tue Jul 11, 2006 6:51 am    Post subject: Re: Input impedance on an instrumentation amp Reply with quote

spam.zachr@gmail.com wrote:

Quote:
I must be missing something obvious... I am working on the front end
of a differential ADC system that is designed to measure battery cell
voltages. I am using an 4052 feeding a AD623. I really need a high
input impedance >1meg. According to the data sheet, when measuring
"floating" input sources (the batteries are definately floating), you
must provide for a bias current path. The recommended circuit uses two
100k resistors from each input to ground. That would make the input
impedance ~200k or less. That's way to low for my application. What am
I overlooking?

The bias current paths are usually provided by the the feedback and input
resistors.

Do you have a schematic ?

Graham
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Winfield Hill
electronics forum Guru


Joined: 05 May 2005
Posts: 1996

PostPosted: Tue Jul 11, 2006 8:50 am    Post subject: Re: Input impedance on an instrumentation amp Reply with quote

Eeyore wrote...
Quote:

Zach wrote:

I must be missing something obvious... I am working on the front end
of a differential ADC system that is designed to measure battery cell
voltages. I am using an 4052 feeding a AD623. I really need a high
input impedance >1meg. According to the data sheet, when measuring
"floating" input sources (the batteries are definately floating), you
must provide for a bias current path. The recommended circuit uses two
100k resistors from each input to ground. That would make the input
impedance ~200k or less. That's way to low for my application. What am
I overlooking?

The bias current paths are usually provided by the the feedback and
input resistors. Do you have a schematic ?

Not in the case of high-Z instrumentation amplifiers, Graham,
where the feedback resistors come after the input buffers (see
the AD623 datasheet).

Zach, are the batteries really floating? I mean, is one end of
your battery-stack grounded, etc? In that case you don't need
to provide another DC bias path, that's provided by the cells.

Also, 100k, etc., bias path resistors are unreasonable, since the
AD623 has only 17nA of bias current. I don't know your battery /
system-power setup, but if the system power is ever off while
the electronics is connected to the battery cells, you can have
the awkward situation of powering the system through the 4052
CMOS switch or the AD623's protection diodes, etc. You may want
series resistors for the battery-cell connections. Also, there
are more advance MUX types that disconnect themselves when the
power is off, or if the input voltages exceed their supply rails.


--
Thanks,
- Win
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Jon
electronics forum addict


Joined: 12 Oct 2005
Posts: 96

PostPosted: Tue Jul 11, 2006 2:54 pm    Post subject: Re: Input impedance on an instrumentation amp Reply with quote

spam.zachr@gmail.com wrote:
Quote:
I must be missing something obvious... I am working on the front end
of a differential ADC system that is designed to measure battery cell
voltages. I am using an 4052 feeding a AD623. I really need a high
input impedance >1meg. According to the data sheet, when measuring
"floating" input sources (the batteries are definately floating), you
must provide for a bias current path. The recommended circuit uses two
100k resistors from each input to ground. That would make the input
impedance ~200k or less. That's way to low for my application. What am
I overlooking?

Thanks,

Zach

Zach,
You must provide a path for bias current. However, the value of the
resistor value to ground depends on the output error that you can
tolerate due to input bias current. The output error will be
approcimately equal to Ib X Rbias X Ac, where Ib is the bias current,
RBias is the summ of the bias resistors, Ac is the closed loop gain.
Regards,
Jon
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spam.zachr@gmail.com
electronics forum beginner


Joined: 11 Jul 2006
Posts: 4

PostPosted: Tue Jul 11, 2006 3:23 pm    Post subject: Re: Input impedance on an instrumentation amp Reply with quote

Quote:
The bias current paths are usually provided by the the feedback and
input resistors. Do you have a schematic ?

Not in the case of high-Z instrumentation amplifiers, Graham,
where the feedback resistors come after the input buffers (see
the AD623 datasheet).

Zach, are the batteries really floating? I mean, is one end of
your battery-stack grounded, etc? In that case you don't need
to provide another DC bias path, that's provided by the cells.

Also, 100k, etc., bias path resistors are unreasonable, since the
AD623 has only 17nA of bias current. I don't know your battery /
system-power setup, but if the system power is ever off while
the electronics is connected to the battery cells, you can have
the awkward situation of powering the system through the 4052
CMOS switch or the AD623's protection diodes, etc. You may want
series resistors for the battery-cell connections. Also, there
are more advance MUX types that disconnect themselves when the
power is off, or if the input voltages exceed their supply rails.


--
Thanks,
- Win


Thank you to everyone that responded.

Let me answer some of the questions:

Rene, I am not sure I understand your suggestion. This is a
differential
system so both inputs on the amp are inputs.

Graham, I am just copying the schematic on page 14 of the datasheet
figure 47c.
http://www.analog.com/UploadedFiles/Data_Sheets/516895375AD623_c.pdf

Win, yes the batteries are completely isolated and need to stay that
way.

I wired up a test circuit and found that things start to bounce around
with ~300k ohms per DC bias resistor. There must be another trick to
get higher input impedances when working with a fully isolated input.

Good point on the power off condition. I will start looking for a more
advanced analog switch. Any suggestions?

Thanks,

Zach
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spam.zachr@gmail.com
electronics forum beginner


Joined: 11 Jul 2006
Posts: 4

PostPosted: Tue Jul 11, 2006 3:50 pm    Post subject: Re: Input impedance on an instrumentation amp Reply with quote

spam.zachr@gmail.com wrote:
Quote:
The bias current paths are usually provided by the the feedback and
input resistors. Do you have a schematic ?

Not in the case of high-Z instrumentation amplifiers, Graham,
where the feedback resistors come after the input buffers (see
the AD623 datasheet).

Zach, are the batteries really floating? I mean, is one end of
your battery-stack grounded, etc? In that case you don't need
to provide another DC bias path, that's provided by the cells.

Also, 100k, etc., bias path resistors are unreasonable, since the
AD623 has only 17nA of bias current. I don't know your battery /
system-power setup, but if the system power is ever off while
the electronics is connected to the battery cells, you can have
the awkward situation of powering the system through the 4052
CMOS switch or the AD623's protection diodes, etc. You may want
series resistors for the battery-cell connections. Also, there
are more advance MUX types that disconnect themselves when the
power is off, or if the input voltages exceed their supply rails.


--
Thanks,
- Win


Thank you to everyone that responded.

Let me answer some of the questions:

Rene, I am not sure I understand your suggestion. This is a
differential
system so both inputs on the amp are inputs.

Graham, I am just copying the schematic on page 14 of the datasheet
figure 47c.
http://www.analog.com/UploadedFiles/Data_Sheets/516895375AD623_c.pdf

Win, yes the batteries are completely isolated and need to stay that
way.

I wired up a test circuit and found that things start to bounce around
with ~300k ohms per DC bias resistor. There must be another trick to
get higher input impedances when working with a fully isolated input.

Good point on the power off condition. I will start looking for a more
advanced analog switch. Any suggestions?

Thanks,

Zach

One other data point...

I was looking at some of TI's in-amps and noticed on their data sheets
they only use a DC bias resistor on one input (not both). So, I tried
that on my test circuit and it seems to work. Just a 100k from the +
input to gnd. Any reason I shouldn't do this?
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Ancient_Hacker
electronics forum Guru Wannabe


Joined: 07 Jun 2005
Posts: 172

PostPosted: Tue Jul 11, 2006 4:21 pm    Post subject: Re: Input impedance on an instrumentation amp Reply with quote

spam.zachr@gmail.com wrote:
Quote:
I must be missing something obvious... I am working on the front end
of a differential ADC system that is designed to measure battery cell
voltages. I am using an 4052 feeding a AD623. I really need a high
input impedance >1meg.

Are the batteries floating? If so, just a little static and you'll
likely exceed the common-mode range of the InAmp. Can you guard the
batteries by having a high value (say 100k) resistor from one battery
terminal to gorund?




According to the data sheet, when measuring
Quote:
"floating" input sources (the batteries are definately floating), you
must provide for a bias current path.

The recommended circuit uses two
Quote:
100k resistors from each input to ground. That would make the input
impedance ~200k or less. That's way to low for my application. What am
I overlooking?


You should only need one resistor to ground, to guarantee a safe
common-mode voltage and to provide a few nanoamps of bias current.

Mots InAmps have input buffer amps, so their input impedance is REALLY
up there. See the spec sheet and calculate how high you can safely go
with the resistor or resistors.


Quote:

Zach
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Ancient_Hacker
electronics forum Guru Wannabe


Joined: 07 Jun 2005
Posts: 172

PostPosted: Tue Jul 11, 2006 4:24 pm    Post subject: Re: Input impedance on an instrumentation amp Reply with quote

spam.zachr@gmail.com wrote:

Quote:
I wired up a test circuit and found that things start to bounce around
with ~300k ohms per DC bias resistor. There must be another trick to
get higher input impedances when working with a fully isolated input.

Bounce is probablyl caused by noise and Ac pickup. You may want to add
a low pass filter at the input, say a 1 megohm resistor in series with
each input and a 5uf poly capacitor to ground. That wil roll off a
whole heck of a lot of 60Hz and random noise.
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tlbs
electronics forum addict


Joined: 28 Apr 2005
Posts: 74

PostPosted: Tue Jul 11, 2006 8:25 pm    Post subject: Re: Input impedance on an instrumentation amp Reply with quote

spam.zachr@gmail.com wrote:
Quote:
I must be missing something obvious... I am working on the front end
of a differential ADC system that is designed to measure battery cell
voltages. I am using an 4052 feeding a AD623. I really need a high
input impedance >1meg. According to the data sheet, when measuring
"floating" input sources (the batteries are definately floating), you
must provide for a bias current path. The recommended circuit uses two
100k resistors from each input to ground. That would make the input
impedance ~200k or less. That's way to low for my application. What am
I overlooking?

Thanks,

Zach

The AD623 looks similar enough to the AD620. We typically use 10Meg
bias resistors on both inputs of our circuits using the AD620 and
AD621. The bias current is low enough that even with 5% bias resistors
it doesn't present a problem at the output.

HTH,
Tom
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spam.zachr@gmail.com
electronics forum beginner


Joined: 11 Jul 2006
Posts: 4

PostPosted: Wed Jul 12, 2006 3:49 am    Post subject: Re: Input impedance on an instrumentation amp Reply with quote

tlbs101 wrote:
Quote:
spam.zachr@gmail.com wrote:
I must be missing something obvious... I am working on the front end
of a differential ADC system that is designed to measure battery cell
voltages. I am using an 4052 feeding a AD623. I really need a high
input impedance >1meg. According to the data sheet, when measuring
"floating" input sources (the batteries are definately floating), you
must provide for a bias current path. The recommended circuit uses two
100k resistors from each input to ground. That would make the input
impedance ~200k or less. That's way to low for my application. What am
I overlooking?

Thanks,

Zach

The AD623 looks similar enough to the AD620. We typically use 10Meg
bias resistors on both inputs of our circuits using the AD620 and
AD621. The bias current is low enough that even with 5% bias resistors
it doesn't present a problem at the output.

HTH,
Tom

The AD620/1 are dual supply and have .5nA of input bias current. The
AD623 uses a single supply has 17nA of input bias current (34 times as
much current). If you take your 10meg resistors and divide them by 34,
you get ~300k ohms. Which is about the max I could use and still get
readings without significant errors. I'm still not sure if I need
resistors on both inputs. My test circuit seems to work fine with just
one, but I'd hate for something bad to happen under different operating
conditions.

Thanks,

Zach
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Ban
electronics forum Guru


Joined: 04 May 2005
Posts: 466

PostPosted: Wed Jul 12, 2006 10:25 am    Post subject: Re: Input impedance on an instrumentation amp Reply with quote

spam.zachr@gmail.com wrote:
Quote:

The AD620/1 are dual supply and have .5nA of input bias current. The
AD623 uses a single supply has 17nA of input bias current (34 times as
much current). If you take your 10meg resistors and divide them by
34, you get ~300k ohms. Which is about the max I could use and still
get readings without significant errors. I'm still not sure if I need
resistors on both inputs. My test circuit seems to work fine with just
one, but I'd hate for something bad to happen under different
operating conditions.

Thanks,

Zach

The battery itself constitutes a very low ohmic serial resistance for almost
all frequencies including DC. So you need only a single resistor, or you can
just ground 1 terminal. There will be only the bias current flowing through
the battery. Why do you use a differential measurement, IMHO you can do as
well with a single ended ADC + opamp.

--
ciao Ban
Apricale, Italy
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Tony Williams
electronics forum Guru


Joined: 01 May 2005
Posts: 374

PostPosted: Thu Jul 13, 2006 6:31 am    Post subject: Re: Input impedance on an instrumentation amp Reply with quote

In article <1152631430.491101.44290@m79g2000cwm.googlegroups.com>,
<spam.zachr@gmail.com> wrote:
]snip]
Quote:
Win, yes the batteries are completely isolated and need to stay
that way.

Battery voltages move slowly, so if you can accept
an update rate of no more than about once each 30
seconds then a truly isolated scanner can be built
with a relay-based flying capacitor system.

RL1a |
Cell+ ---/\/\-----o\ o-+---->ADC
R 470k \ |
o
|
C === 0.1u
|
o
R 470k / |
Cell- ----/\/\----o/ o----+->0v
RL1b |

A 4-channel scanner would require 4 2PCO relays, wired
as above, with all 0v and ADC outputs bussed together.

To measure a cell voltage, energise that relay to swing
the capacitor over to the ADC, measure the voltage, put
the relay back. With a 0.1uF cap, each relay should be
energised for no more than about 10mS.

Once each C has been charged for the first time the load
presented to each cell will be the leakage of the C.

--
Tony Williams.
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