# LF398 Sample and Hold Circuit

I'm having a problem designing a circuit to interface an LF398. On it's data sheet it says that it needs +5 and -5 volts, but I only have +5.

My questions are:

1. Can I put V+ on 5V and V- on ground and still have a viable circuit and how would this constrain my input values?

2. Is there a way I can get both 5V and -5V without using another IC?

As a plan B, I could use an voltage inverter to get the -5, but this would require another dedicated IC. The samples that I'm trying to hold will be in the -1 to 1 volts range.

A little background: I'm building a pulse oximeter for a class project. We have 2 flashing LEDs that reflect off of skin into a photodiode that is connected to a microcontroller for processing. The series of events should be: LED flash -> Photodiode -> S&H -> Micro. The S&H IC should sample when that specific LED is high.

RM: LF389 does nort exist - changed to LF398

-
Wow... I recently did this myself. Didn't use a monolithic SHA chip though. Now working on possible improvements. –  Ben Voigt Nov 14 '12 at 21:18

The data sheet clearly says "operates from +/- 5V to +/- 18V supplies"

ie it needs a minimum of 10V from V+ to V- to operate (and a maximum of 36V).
Why would you expect it to operate on only 5V?

It does not care if you ground V- and use +10V or ground V+ and use -10V but you need 10V minimum supply.

Note that at the top of each data table it says that the figures apply for
V- + 3.5V < Vin < V+-3.5V.
ie signals must not be closer than 3.5V from either supply rail. So on a +/- supply you get +/- 1.5V signal range.

There is a small chance that it will run on slightly less voltage than specified, but if it did there would be no guarantee re performance.

The world is awash with ways of making dual supplies. Two batteries, voltage divider from one supply, two winding power supply. ... . That's a reasonably nice IC that you are using - it deserves to be fed properly. You say that you only have +5V. Why? How?

About the cheapest build your own solution is to use an MC34063 - various application circuits with PCB layout in datasheet. But you can buy modules for a few dollars on ebay that do what you want.

-
I'm trying to run it through USB, making it small enough to put on a wrist. We're trying to get all of the ICs from free samples to reduce costs and I chose the LM389 because it was one of the only one's with free samples. Kind of got it without thinking. –  Raufio Oct 23 '12 at 3:46