Posted by Charles Linquist April 24, 2011
You should be able to use a fairly large, low-leakage capacitor on the
input of the A/D. This will, of course, limit how fast you can see
changes in the input voltage, but if you are simply measuring DC, then
that shouldn't be a problem. The capacitor will charge to the divided
voltage, and provide current while the A/D is sampling. Accuracy and
low power at the same time.
The cap will also filter out any noise that is induced into the
high-impedance voltage divider.

Charles Linquist.
Posted by Chad Russel April 24, 2011
Or you could use higher value resistors with a very low power voltage follower, but that could be expensive if you need accuracy or speed, and could end up using more power in the end.  If you can live with lower speed or lower accuracy the 10k limit can be raised.  There should be a formula for sample time versus input resistance.
My software has no bugs, only undocumented features.
Posted by Dave March 19, 2011
Mouser and DigiKey are expensive on shipping for small quantities, as are
Farnell. Last time I checked Maplin only had E24 (I use them often). Rapid
Electronics may be better, but I haven't checked. I drive past RS Manchester
most days and if I call and collect there are no delivery charges, and I can
get most things the day after I order....

Dave
Posted by cdb March 19, 2011
Check out Rapid, they are likely to be cheaper on the components.

0805 1% 46p/100

www.rapidonline.com

Colin

:: I am in the UK.but on
:: checking on Radio Spares I can get a wide range of values in
:: surface mount,
:: although some are 80p each in packs of 10 so thats about $1 each.
:: I hadn't
:: realized that was possible as I normally only use wired
:: components..
--
cdb, c...@btech-online.co.uk on 20/03/2011

Web presence: www.btech-online.co.uk
Posted by Jeff Cooper March 19, 2011
> I am in the UK.but on checking on Radio Spares I can get a wide range
> of values in surface mount, although some are 80p each in packs of 10
> so thats about $1 each. I hadn't realized that was possible as I
> normally only use wired components..

I've found a surprising range of values at the distributors I mentioned
(Mouser and DigiKey). You might check out Maplin, Farnell, and the like.
They may have minimum order requirements but at least it gives you an
idea. Also, never underestimate eBay.

> Thanks, very useful
> Dave

Best Wishes,
Coop, AA1WW
Posted by Max March 19, 2011
Thanks for all of the replys. Now to check the suppliers and see what values they have that will work.

Max
Posted by Dave March 19, 2011
> Firstly because of the way the preferred values work its impossible to
> get a ratio 4:1 as needed to get an analogue divide by 5. So there is
> 1K but only 3.9 or 4.1.

You can get a lot of values these days. Check out Mouser or DigiKey
or any other full line distributor (I happen to be in the US).

I am in the UK.but on checking on Radio Spares I can get a wide range of
values in surface mount, although some are 80p each in packs of 10 so thats
about $1 each. I hadn't realized that was possible as I normally only use
wired components..
> Secondly , when I saw this I thought the data sheet would say what the
> resistance or impedance of the the input were but I can't find it
> anywhere. All it says is that the the impedance of the voltage should
> be less than 10K which seems very low,

As Les inferred, if you can leave the sample and hold open for more
you might be able to read a higher impedance source.

Ok I had forgotten it had a cap and used sample and hold, so that makes
sense now...

> I have a PIC based Ham Radio test tool that measures battery voltage
> and that uses 16k/3.9k as the dividers to give a divide ratio of 5.1 ,
> but that's >10k

Not actually. Again, Les has it right here. You need two resistors of
value Rx and 4Rx to make a 5:1 voltage divider. The source impedance
seen by the ADC will be their Thevenin equivalent or (4 * Rx^2)/5Rx or
0.8Rx which wants to be <= 10K. Ergo Rx must be <= 12.5K and 4Rx
is 50K.

(16K * 4K)/(20K) ~= 3.2K so you're well below the constraint.

OK I follow that, after a little googling an re-reading..

Max, I recently designed a sophisticated battery charger/monitor/UPS
using a PIC24H family part and need a high impedance source for
measuring battery voltage. Along with Les' suggestion on considering
a longer sample and hold period to charge the PIC16F877's internal
sample capacitor, you could put a 0.1uF cap on the input. This cap
would charge to the sample voltage and then at sample time would
provide extra current to charge the PIC's internal sample cap.

Some of this stuff may be discussed in a Microchip document called
something like "Midrange CPU Family Family Reference". It's always
seems to take me a couple of tries to find it at Microchip's website but
it probably has the background info on the A/D section.

I think I have that on the main PC....

Coop, AA1WW

Thanks, very useful
Dave
Posted by Jeff Cooper March 19, 2011
> Firstly because of the way the preferred values work its impossible to
> get a ratio 4:1 as needed to get an analogue divide by 5. So there is
> 1K but only 3.9 or 4.1.

You can get a lot of values these days. Check out Mouser or DigiKey
or any other full line distributor (I happen to be in the US).

> Secondly , when I saw this I thought the data sheet would say what the
> resistance or impedance of the the input were but I can't find it
> anywhere. All it says is that the the impedance of the voltage should
> be less than 10K which seems very low,

As Les inferred, if you can leave the sample and hold open for more
you might be able to read a higher impedance source.

> I have a PIC based Ham Radio test tool that measures battery voltage
> and that uses 16k/3.9k as the dividers to give a divide ratio of 5.1 ,
> but that's >10k

Not actually. Again, Les has it right here. You need two resistors of
value Rx and 4Rx to make a 5:1 voltage divider. The source impedance
seen by the ADC will be their Thevenin equivalent or (4 * Rx^2)/5Rx or
0.8Rx which wants to be <= 10K. Ergo Rx must be <= 12.5K and 4Rx
is 50K.

(16K * 4K)/(20K) ~= 3.2K so you're well below the constraint.

Max, I recently designed a sophisticated battery charger/monitor/UPS
using a PIC24H family part and need a high impedance source for
measuring battery voltage. Along with Les' suggestion on considering
a longer sample and hold period to charge the PIC16F877's internal
sample capacitor, you could put a 0.1uF cap on the input. This cap
would charge to the sample voltage and then at sample time would
provide extra current to charge the PIC's internal sample cap.

Some of this stuff may be discussed in a Microchip document called
something like "Midrange CPU Family Family Reference". It's always
seems to take me a couple of tries to find it at Microchip's website but
it probably has the background info on the A/D section.

Coop, AA1WW
Posted by Leon Heller March 19, 2011
On 19/03/2011 12:07, Dave wrote:
> Firstly because of the way the preferred values work its impossible to
> get a ratio 4:1 as needed to get an analogue divide by 5. So there is 1K
> but only 3.9 or 4.1.
> Secondly , when I saw this I thought the data sheet would say what the
> resistance or impedance of the the input were but I can't find it
> anywhere. All it says is that the the impedance of the voltage should be
> less than 10K which seems very low,
> I have a PIC based Ham Radio test tool that measures battery voltage and
> that uses 16k/3.9k as the dividers to give a divide ratio of 5.1 , but
> that's >10k
> So as you can see its a complete mess......
> You might want to consider 2k and 10K which gives a divide by 6., or
> even 16K and 2x2k in series to give divide by 5.

Impedance must be below 10k for proper operation of the ADC.

Leon
--
Leon Heller
G1HSM
Posted by Dave March 19, 2011
Firstly because of the way the preferred values work its impossible to get a
ratio 4:1 as needed to get an analogue divide by 5. So there is 1K but only
3.9 or 4.1.

Secondly , when I saw this I thought the data sheet would say what the
resistance or impedance of the the input were but I can't find it anywhere.
All it says is that the the impedance of the voltage should be less than 10K
which seems very low,

I have a PIC based Ham Radio test tool that measures battery voltage and
that uses 16k/3.9k as the dividers to give a divide ratio of 5.1 , but
that's >10k

So as you can see its a complete mess......

You might want to consider 2k and 10K which gives a divide by 6., or even
16K and 2x2k in series to give divide by 5.

Dave
G4UGM