Why not the LM339 instead of LM311 in AADE LC Meter clones?

[KamalS]

I was looking at the various AADE LC Meter clones on the net and found that all of them uses the LM311 Comparator without exception.

My question is - why not the more popular LM339?

Here are the parameters I think matters in selection of the opamp in this LC Meter:

1. Input bias current ( lower the better - reduces loading on the LC Tank )
2. Output ( sink? ) current
3. Slew rate

( if there are more parameters to be taken into cinsideration, let me know. For eg. input gate capacitance of the comparator? )

If these 3 parameters matter, here are the specs consolidated from TI, ST and Phillips datasheets:

LM339:
1. 25 - 400nA
2. 6 - 16mA
3. Not there in any datasheet!

LM311:
1. 100 - 250nA
2. Not there in any datasheet!
3. Not there in any datasheet!

The LM311 features two pins for balance/strobe capability, but the pins are shorted - this makes me think that this feature is not used at all, in that case.

From this information ( I assume the slew rate is much higher than 5V/uS for both but have no way of knowing ), it seems the 311 in the AADE could be replaced, withotu any change by the 339?

If I am overlooking something, do point it out!

 

[schmitt trigger]

The LM311 is faster, and with the strobe pins shorted together, faster still.
Aditionally, the input/output characteristics of the LM311 are more flexible. For instance, the input can be biased with a supply of +/-15 volts, yet the output can be GROUND REFERRED.

Go to National.com They invented both devices, and their spec sheets are very informative.

 

[ericgibbs]

hi,
This is an important difference, if the output requires to sink 50mA the LM311 can do that, the LM393 cannot..

 

[crutschow]

Here are the parameters I think matters in selection of the opamp in this LC Meter:

1. Input bias current ( lower the better - reduces loading on the LC Tank )
2. Output ( sink? ) current
3. Slew rate

The LM311/339 are comparators, not op amps. A comparator is designed to operate open loop and give a digital (binary) output. An op amp can be used as a comparator, but operate relatively slowly for that function. They are normally used with feedback in a linear or quasi-linear mode.

Item 3: Slew rate. This can be important in op amp linear applications, but is not usually specified for comparators. Comparators specify response time, which is the time from where the inputs cross the compare point to the time the output changes states.

 

[vsmGuy]

It would seem that the 339 might have spurious oscillations after ~700kHz.

Try both of them out and see.

 

[KamalS]

Could you tell me how the response time of a comparator affects its freq. response?

Links/books where this is clearly mentioned and illustrated would help me too

The LC Meter typically creates freq. between a few kHz to ~1Mhz at the max ( = 1uS or 1000nS )

For a 5V logic, this requires a comparator having :

1. response time of at least 1000nS
2. slew rate of > 5V/uS

Where does the 339 fail?

I think I should link to some of the designs - as not everyone knows about the AADE meter.

Here is a link:

The LC Meter typically creates freq. between a few kHz to ~1Mhz at the max ( = 1uS or 1000nS )

For a 5V logic, this requires a comparator having :

1. response time of at least 1000nS
2. slew rate of > 5V/uS

Where does the 339 fail?

I think I should link to some of the designs - as not everyone knows about the AADE meter.

Here is a link:

**broken link removed**

 

[KamalS]

In my LC Meter, I would use the "counting method" of finding the freq.

That is, I would, for a fixed period of time, let the exciting signal increment a timer.

For example, I would let the output of the LC oscillator interrupt a pin for 3 secs, and in the ISR for that interrupt, I would keep a track of how many times the interrupt was fired.

Here is the specifications of the AADE meter:

The resonant freq of the 68uH, 680pF AADE LC Osc is ~ 740kHz

For the nominal values of L1 (68 uH) and C1 (680 pF) an increase in L of 1 nH (.001 uH) or an increase in C of .01 pF produces a frequency change of slightly more than 5 Hz ( 5.439923 ).

A 0.2 second measuring period can resolve 5 Hz and therefore .001 uH or .01 pF.

For small values the frequency of operation (test frequency) is about 750 KHz decreasing to about 60 KHz at .1 m F's or 10 mH's and about 20 KHz at 1 m F or 100 mH's.

A 10mH L will be tested @~ 60kHz
A 1nF C will be tested @~ 480kHz

For my version of the meter, I would use the LM339, L = 100uH, C = 2nF.

The sampling period has to be increased to 3 secs instead of 0.2 secs that could have been possible with the LM311, making the meter 10 times slower.

For small values the frequency of operation (test frequency) is about 356 KHz decreasing to about 49 KHz at .1mF's, 35kHz for 10mH's, 15KHz at 1mF, 11kHz for 100 mH's.

A 10mH L will be tested @~ 35kHz
A 1nF C will be tested @~ 290kHz

Does the specs of my meter look good?