Sample and Hold Analog Voltage

[aman]

I am designing an electronic interface to SCADA. The SCADA typically
takes 4-20 mA input for logging data to a central database. So I need
to convert my voltage output from my instrument/device into current
using a V/I convertor(like AD1B22).
My instrument has just one analog out but I need 4 lines to interface
to SCADA. For this I am using a 4:1 MUX. Now since these 4 lines should
hold valid data at all times I need to latch in voltage values before
converting them to current outputs. I was thinking of using sample and
hold circuit to latch analog voltage values. When I looked at sample
and hold circuit like AD585, the data sheet talks about droop
rate(change in output voltage with time due to leakage capacitive
current).

So I am thinking that the only way to latch analog voltage is to
convert it into digital, then latch the voltage and convert it back to
analog. Am I thinking on correct lines or not ? Any other ideas ?

 

[John Fields]

I am designing an electronic interface to SCADA. The SCADA typically
takes 4-20 mA input for logging data to a central database. So I need
to convert my voltage output from my instrument/device into current
using a V/I convertor(like AD1B22).
My instrument has just one analog out but I need 4 lines to interface
to SCADA. For this I am using a 4:1 MUX. Now since these 4 lines should
hold valid data at all times I need to latch in voltage values before
converting them to current outputs. I was thinking of using sample and
hold circuit to latch analog voltage values. When I looked at sample
and hold circuit like AD585, the data sheet talks about droop
rate(change in output voltage with time due to leakage capacitive
current).

So I am thinking that the only way to latch analog voltage is to
convert it into digital, then latch the voltage and convert it back to
analog. Am I thinking on correct lines or not ? Any other ideas ?

 

[petrus bitbyter]

I am designing an electronic interface to SCADA. The SCADA typically
takes 4-20 mA input for logging data to a central database. So I need
to convert my voltage output from my instrument/device into current
using a V/I convertor(like AD1B22).
My instrument has just one analog out but I need 4 lines to interface
to SCADA. For this I am using a 4:1 MUX. Now since these 4 lines should
hold valid data at all times I need to latch in voltage values before
converting them to current outputs. I was thinking of using sample and
hold circuit to latch analog voltage values. When I looked at sample
and hold circuit like AD585, the data sheet talks about droop
rate(change in output voltage with time due to leakage capacitive
current).

So I am thinking that the only way to latch analog voltage is to
convert it into digital, then latch the voltage and convert it back to
analog. Am I thinking on correct lines or not ? Any other ideas ?

The AD585 is a fast SAHS for fast applications. For you application an LF198
or LF398 is more suitable. This ones have a droop rate of 5mV/min. If you
keep your sample frequency at some kHz, this drooprate is neglectable.

petrus bitbyter

 

[aman]

My sample frequency is not even KHz, it is Hz. I need a sample every 5
minutes. So if I use LF398 I dont need to latch data at all. Is that
true ?

 

[aman]

Ok I got it. So if I sample every 5 min, I need to consider if droop of
25mv is acceptable to me or not. Correct ?

 

[petrus bitbyter]

Ok I got it. So if I sample every 5 min, I need to consider if droop of
25mv is acceptable to me or not. Correct ?

Yes.

But I don't understand whether you need only one sample every 5 min or if
you can get only one sample during that time. That makes a lot of
difference. If you can sample several times/minite you can still use that
LF398.

If you can get only one sample per 5 minutes you have to consider whether or
not 25mV is acceptable. In a practical circuit the result will be better but
you can't count on that. You may find SAHSs with better droop rate. Analog
Devices offers one they claim to be "droopless", the AD5533. You may want to
use that one until you see its pricetag. It's also a little bit overkill as
it offers 32 channels. It just does what you suggested: AD-conversion,
storing the digital data and DA again for the output. It also claims 0.018%
accuracy so I guess they use a 14 bit or better AD converter.

So if you want to go that way you'll have to look what accuracy you need. If
your signal is in the 0 to 5V range, an 8 bits AD-converter has a 19mV
conversion error already. That's in the same order of magnitude the LF398
offers. If you have to do better, I'd take one good enough AD-converter and
Multiplex the inputs using (reed)relay. Store the output in flip flops or
registers (the old LS364 comes to mind, but you may be able to do better
these days). Then it takes one DA converter per channel.

petrus bitbyter

 

[aman]

Thanks Petrus.

I got the point.

-Aman

 

[Jasen Betts]

My sample frequency is not even KHz, it is Hz. I need a sample every 5
minutes. So if I use LF398 I dont need to latch data at all. Is that
true ?

it's not how often you sample it's how long it takes te preform each sample
that determines the suitability of the sample-and-hold circuit.

after you've converted it to digital what the sample-and-hold circuit does
is immaterial.

Bye.
Jasen