novice: op amp as voltage comparator

[fuzzymonkey]

Hi,
I'm using this (OPA134)
http://www.jaycar.com.au/products_uploaded/ZL3972.pdf
to determine whether the voltage from a pot or the output from a
digital-analog converter is higher. No matter what voltage i put to the
inputs, the output always stays at around 0.6v. Power supply is 5v from USB
port.

I've double checked the connections and measured the voltages directly at
the input pins..

Have somehow fried the IC or have i just misunderstood something?

thanks for any help!
-Anthony

 

[Swanny]

Hi,
I'm using this (OPA134)
http://www.jaycar.com.au/products_uploaded/ZL3972.pdf
to determine whether the voltage from a pot or the output from a
digital-analog converter is higher. No matter what voltage i put to the
inputs, the output always stays at around 0.6v. Power supply is 5v from
USB port.

I've double checked the connections and measured the voltages directly
at the input pins..

Have somehow fried the IC or have i just misunderstood something?

thanks for any help!
-Anthony

You haven't exactly described how you have it wired up. That might make
a difference ...

 

[fuzzymonkey]

on a breadboard..
Do you mean a schematic of the entire circuit?

 

[Trevor Wilson]

on a breadboard..
Do you mean a schematic of the entire circuit?

**Of course. We need to see what you've done wrong. BTW: The 5 Volts
available from a USB port is at the very low end of what is usable for that
particular chip.

 

[Swanny]

on a breadboard..
Do you mean a schematic of the entire circuit?

Yes. A schematic or description of how you are using it may help.

 

[David L. Jones]

on a breadboard..
Do you mean a schematic of the entire circuit?

A photo of your breadboard setup is best and easiest way we can see exactly
what's happening.

Ideally you should be using a comparator for this, like an LM311 for
example.

Dave.

 

[fuzzymonkey]

A photo of your breadboard setup is best and easiest way we can see

exactly what's happening.

Ideally you should be using a comparator for this, like an LM311 for
example.

Dave.

ok, a picture makes sense. But i can't get a clear one with my crappy camera
phone even with flash.

I'll try a higher supply voltage but i read just over 5v from USB port and
5.00v exactly on the power supply pins of the IC.

I can get a LM311 in a few days.

What i'm trying to achieve and how i'm doing it:
perform an analogue to digital conversion on a sine wave signal with a max
frequency of about 266Hz. Originally started with a dedicated ADC IC but at
the highest rated clock speed the sample rate was still too slow for what i
needed.

Then tried using DAC chip driven by a counter and connected to comparator to
act as Analog-digital converter. For some reason the DAC chip outputted
completely weird voltages (i've checked the counter outputs that pass to DAC
chip as counter ic was clocked) so i've temporarily replaced DAC chip with a
R/2R resistor network.

so i'm just left with weird op amp behavior

thanks for suggestions so far!

 

[David L. Jones]

ok, a picture makes sense. But i can't get a clear one with my crappy
camera phone even with flash.

You don't have a real camera?
Most likely a low light (indoors) problem, take it outdoors on a sunny day
and try again.

What i'm trying to achieve and how i'm doing it:
perform an analogue to digital conversion on a sine wave signal with
a max frequency of about 266Hz. Originally started with a dedicated
ADC IC but at the highest rated clock speed the sample rate was still
too slow for what i needed.

What ADC would be too slow for 266Hz?

Then tried using DAC chip driven by a counter and connected to
comparator to act as Analog-digital converter. For some reason the
DAC chip outputted completely weird voltages (i've checked the
counter outputs that pass to DAC chip as counter ic was clocked) so
i've temporarily replaced DAC chip with a R/2R resistor network.

You are jumping through hoops doing it that way. Try a real ADC again.

What is reading the data and controlling this?, a microcontroller perhaps?

Dave.

 

[Franc Zabkar]

**Of course. We need to see what you've done wrong. BTW: The 5 Volts
available from a USB port is at the very low end of what is usable for that
particular chip.

.... and the Common-Mode Voltage Range is (V–)+2.5 to (V+)–2.5.

- Franc Zabkar

 

[Franc Zabkar]

I'll try a higher supply voltage but i read just over 5v from USB port and
5.00v exactly on the power supply pins of the IC.

I can get a LM311 in a few days.

I think you need a device that goes rail-to-rail on the input side.

Maxim Micropower, Low-Voltage, UCSP/SC70, Rail-to-Rail I/O
Comparators:
http://www.maxim-ic.com/getds.cfm?qv_pk=1698

"The MAX985/MAX986/MAX989/MAX990/MAX993/MAX994 single/dual/quad
micropower comparators feature low-voltage operation and rail-to-rail
inputs and outputs. Their operating voltages range from 2.5V to
5.5V ..."

Free samples:
https://shop.maxim-ic.com/storefront/searchsample.do?event=Sample&menuitem=Sample&Partnumber=MAX986

Be aware that some comparators have open-collector or open-drain
outputs which means you will need to add a pullup resistor.

- Franc Zabkar