op amp Bandwidth

[Pao]

Hi

Im tinkering with a a r2r ladder circuit and at the op amp part im using a LM324. Sometimes I experience distortion. Im using a LM324 cause i also use the other as a non-inveritng op amp.

“As a general rule the op amp bandwidth should be as small as possible (i.e. a low pass filter) to pass only the expected changes in the digital input, any excess bandwidth only adds noise to the output.”

Got any suggestion as to what op amp I can use as a substitute for the LM324 if possible with the same pin configuration as LM324

Thanks in advance

Pao

 

[Nigel Goodwin]

Hi

Im tinkering with a a r2r ladder circuit and at the op amp part im using a LM324. Sometimes I experience distortion. Im using a LM324 cause i also use the other as a non-inveritng op amp.

“As a general rule the op amp bandwidth should be as small as possible (i.e. a low pass filter) to pass only the expected changes in the digital input, any excess bandwidth only adds noise to the output.”

Got any suggestion as to what op amp I can use as a substitute for the LM324 if possible with the same pin configuration as LM324

I'm sure Audioguru will be along to tell you that the LM324 is going to be distorted - and it isn't a very good chip for what you want.

I've no idea of the pinouts, but you might try looking at the TL074 or similar?.

 

[audioguru]

The old, old, old and cheap LM324 has a very low bandwidth and a high noise level. Its output can have max swing up to only 6kHz for a sine-wave and up to only about 600Hz for a square-wave. It is low power so it also has crossover distortion.

A TL071 single, TL072 dual or TL074 quad opamp has 100kHz bandwidth for sine-waves, is low noise for audio systems and has very low distortion. They are very popular so they are inexpensive.

The LM324 works with a supply voltage as low as 3.0V, can have its inputs work at its negative supply voltage and has an output that can go down to nearly its negative supply voltage. The TL-series opamp can't.

Upload your schematic for us to see if it needs the LM324 kind of opamp or if it will work with the better one.

 

[Hero999]

Let's not rule out the posibility of using a CMOS op-amp.

 

[Pao]

Is it ok for Vcc- (pin11) on TL074 to connect to ground similar to the LM324?

 

[audioguru]

The input and output voltages of your opamps go to ground (the opamp's negative supply voltage). Therefore a TL074 won't work in your circuit.
The LM324 is "a single supply opamp" but it is distorted and slow.

I have used MC3317x single supply opamps with a bandwidth of 35kHz and no distortion, and I have used MC3407x single supply opamps with a bandwidth of 100kHz and no distortion. The 4-opamp versions would probably work well in your circuit.

The pin numbers of an LM324, TL074, MC33174 and MC34074 are the same.

 

[Hero999]

What's the maximum frequency you're planning to use this at?

 

[Pao]

1hz and 250hz

 

[audioguru]

The LM324 works fine at such low frequencies but might have a little noise and crossover distortion.
Explain and sketch what the distortion is like.

 

[Hero999]

A LM324 can handle that so the low bandwidth won't be a problem.

How about noise, what's the minimum voltage you'll be measuring?

 

[epilot]

The input and output voltages of your opamps go to ground (the opamp's negative supply voltage). Therefore a TL074 won't work in your circuit.
The LM324 is "a single supply opamp" but it is distorted and slow.

I have used MC3317x single supply opamps with a bandwidth of 35kHz and no distortion, and I have used MC3407x single supply opamps with a bandwidth of 100kHz and no distortion. The 4-opamp versions would probably work well in your circuit.

The pin numbers of an LM324, TL074, MC33174 and MC34074 are the same.

Sorry for off topic,
audioguru,
Do you want to say that some op amps are designed to work with a single supply and some are designed to work only with negative supplies?
By the way what (V/2) means for an op amp? does it mean ground or negative supply?

 

[audioguru]

Do you want to say that some op amps are designed to work with a single supply and some are designed to work only with negative supplies?
By the way what (V/2) means for an op amp? does it mean ground or negative supply?

Single supply opamps have inputs that work fine at the negative supply voltage (ground in a single supply system) and their outputs are able to go close to the negative supply voltage.
Most opamps have positive and negative power supply voltages or use a voltage divider to give a V/2 (half the single supply) reference voltage. Their inputs and outputs can't go within about 2V from the negative supply voltage.

 

[Pao]

A LM324 can handle that so the low bandwidth won't be a problem.

How about noise, what's the minimum voltage you'll be measuring?

On the sawtooth output at 5V and downwards the Lm324 doesnt seem to produce a sawtooth at all, all I get is a DC voltage.

And at times it produces a expected result and sometimes it produces an odd result (see atachment) that why maybe distortion?

 

[audioguru]

I think the old LM324 opamp is too slow to keep up with the fast negative-going portion of the sawtooth.
Slow down the fall-time of the waveform or use faster opamps.