LM358 variable voltage ?

[Dimbulb]

I am interested in making a battery powered
voltage source.

I want to use an LM358 powered by four AA batteries.

I have a stable 0.002 Volts reference.
How can I get a variable output from
.002 to 5.00 volts ?

Thankyou

[Chris Smith]

A pot and a battery will get you that.

Are you looking for a self adjusting/ regulating source?

[Dimbulb]

The stability and precision of the op amp is needed. it uses a reference of 0.002 volts. My question is to gather useful hints in making a 1X to 250X single supply LM358 instument amplifier using batteries.

The 5 volt supply is regulated
and the reference is well regulated.
I am not sure if self adjusting is what I am trying to do or not ? possibly ?

I would like to turn a pot to adjust the voltage.
But I want the low voltage op amp to use the reference to correct for battery drop and other changes such as in ambient temperature.

Thankyou for your reply

[rshayes]

You want a higher reference voltage.

In theory, a 2 millivolt reference could be amplified up to higher values. In practice, amplifier offset voltage is a problem, since it also gets amplified.

The offset voltage for the LM358 is limited to 7 or 3 millivolts (depending on the grade of part). This is 150 percent to 350 percent error with a 2 millivolt reference.

Since the LM358 is a dual unit in an 8 pin package, there is also no simple way of adjusting the offset to zero.

There are better op amps available, but an offset of 100 microvolts will still result in 5 percent errors.

With a 6 volt supply, you are probably better off generating a 2.5 volt reference and then attenuating the voltage down rather than amplifying it up. The attenuator is more stable and should have no offset (except possible thermoelectric voltages in the microvolt range).

[Chris Smith]

There are soo many designs to chose from.

A high Z adjustable gain [voltage follower] DC instrument amp would probably be my starting point. The Voltage reference starting point and a precision ten turn pot [or more] would allow you to trim the input with almost no load there to affect your out put and the op amp does the rest.

The LM x58 series would not be my first choice but you probably chose it for the single supply function and availability.

Instrument op amps and voltage followers are the data you need to be following, but my book contains several thousand pages so trying to pick just one candy from the candy store is hard to do.

My book is also old so Im sure there are newer models but the LH0084 has a Digitally controlled Programable Gain Instrument amp Diagram from National that may or may not be a good starting point for your reference? [P 4-37]

There are soo many ways to make adjustments to an op amp that listing just one may not be the best for your design. The off set on some amps can be large enough to set your total spread, where as just using a pot, resistor, and voltage source as a voltage divider and feeding it into the input also might do the trick with the op amp having a gain set to 10, 50 or what ever works best [most stable] for the accuracy you need. You need a op amp book full of design notes to best lead you to your goal.