# Using a single power supply for dual supply "Sample and Hold"

Discussion in 'Electronic Basics' started by aman, Oct 24, 2005.

1. ### amanGuest

Sample and hold IC LF198 operates from +-5V to +-18V. I am assuming
that if I bias it using +5V and GND single supply, the only difference
will be that it would just be able to handle rail to rail voltages(that
is, only positive voltages from 0V to 5V).

Is my assumption correct ?

-Aman

2. ### John PopelishGuest

Sorry, no. The device needs between 10 and 30 volts across its supply
pins in order to function correctly. Where ground is between these
voltages is just a matter of input and output signal swing capability.

This detail is hidden in a cryptic comment on the data sheet:
http://www.ee.washington.edu/stores/DataSheets/linear/lf398.pdf

On page 2, directly under the section title "Electrical
Characteristics" it says, "The following specifcations apply for -VS +
3.5 <= VIN +VS -3.5..."

I think that means that all the specifications apply only if the
signal voltage (Vin) is 3.5 or more volts more positive than the
negative supply rail voltage (-VS) and also 3.5 or more volts more
negative than the positive supply rail voltage (+VS).

So the device might have a very small signal range with a positive and
negative 3.5 volt supply (or some combination that adds up to 7 volts,
but they don't actually say it works for any supply total less than 10
volts, total.

3. ### Jasen BettsGuest

according to the above it wants atleast 10V between it's VSS and VDD

+12 (or +10) and 0 could probably work - unless it needs a ground too.

Bye.
Jasen

4. ### amanGuest

Ok now these make sense. So if I have to use input of 0 to 5V, I would
probably have to use power supply like -3.5 and +8.5 atleast. Correct ?

5. ### John PopelishGuest

Yes. That would be the minimum that is covered by the spec sheet.
Selected individual units may do a bit better than that, but it is not
guaranteed.

Thanks