Re: LM358 op-amp

On Apr 17, 7:45 am, Allen Bong <allenbsf6...@xxxxxxxxx> wrote:
On Apr 17, 12:42 am, "Bob Monsen" <rcmon...@xxxxxxxxx> wrote:





"Allen Bong" <allenbsf6...@xxxxxxxxx> wrote in message

news:156490eb-d94f-49fa-a6ed-aca020d20b0f@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

I have constructed a circuit as below to convert the input of 12 to
24V as 0 to 12V and show the voltage on a DC volt-meter.  The
schematics is shown below:

   VCC +24V
    +
    |
   .-.
   | |               +24V
2k2| |               VCC
   '-'                +
    |                 |
    |       10K     |\|
    | +12V-|___|-+--|-\LM358
    |  ___       |  |  >---+-----+
 IN +-|___|---+-----|+/    |     |
    |  10K    |  |  |/|    |     |
    |         |  |   ===   |     |
   .-.        |  |   GND   |     |
   | |        |  |   ___   |     |
2k2| |       .-. +--|___|--+    /+\
   '-'       | |     10K       (   )
    |     10K| |           VOLT \-/
    |        '-'                 |
   ===        |                  |
   GND        |                 ===
             ===                GND
             GND

(created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de)

I have the +12V reference taken from a TL431.

When the input voltage at "IN" is +12V I was not able to get a 0V on
the voltmeter.  Instead I am getting 0.58V at the output.  Does this
has something to do with the CMRR of the chip?  How would I solve
this problem?  Would changing the op-amp to LM748 and connecting a
pot between the balance inputs of the LM748 help to reduce the
voltage to O?

Thanks In Advance!

Allen

The LM358 includes a 50uA pull-down on the output, to get you to 0V.
However, you are pulling up on the output (with the reference through the
two 10k resistors) using 600uA. So, increase all the 10k resistors to 1MEG.
That way, your pull-up will only be 6uA, and it'll be able to get all the
way down to 0V. You can also increase the divider (which uses 2.2k
resistors) to using 1MEG resistors.

When you run into this kind of problem, go to the manufacturer, and see if
they have a data*** for the device. They often include a schematic in the
data***, and using it, you can sometimes figure out what's up.

Regards,
 Bob Monsen- Hide quoted text -

- Show quoted text -

Hi Bob,

Thanks for your good advice. I actually read through the data*** and
the application notes, but there are a lot of things in the data***
I dont understand.  Now reading the data*** again, I see the 50uA at
the 'Output Current Sink' at Vo = 200mV and V+ = 15V.

I'll try it out tonight and report the result here.

Thank you again for helping!

Allen- Hide quoted text -

- Show quoted text -

I have changed all the 10K resistors to 1 Mohm and the circuit worked
as expected even though I kept the voltage divider at 2.2K (there is a
reason I can't change the value of these 2 resistors). I will solder
it up onto a PCB and test it for a few days to observe its
stabilities.

In order to learn more about this op-amp, I am also looking into the
"supplying negative voltage into the V- using a 555" method. I will
ask more questions if I bump into more problems.

Thanks very, very much !

Allen
.