Clarification Ultrasonic Sensors..!!!

[AVR_Craig]

Hi Nigel,

I did read it - yesterday.
This all started two months or so ago. I was trying to paint a bigger picture to get all the details in.
Please read it again. The first 3/4 of the post is how I achieved failure.

In addition to reading Audioguru's post about the 358, I read your posts about filters in the amp stage. That is why one of my two questions was:

"Is it possible to tell from the schematic if the op-amp is tuned for a certain frequency?"

The schematic doesn't mention a frequency and has no values for the 567's resistor and cap to set frequency. I don't remember why I chose 40 khz, but I can use whatever is best, or I can start over with a known good circuit.

 

[audioguru]

Hi AVR Craig,
Your terrible circuit doesn't work.
It is designed for a different kind of opamp (LM3900).
Its ordinary old opamps are too slow and are incorrectly biased.
The opamps are not tuned, the NE567 is tuned.

 

[AVR_Craig]

Thanks Audioguru.

That is not my circuit, I found it online and it never worked.

I ended up using the 358 as a 2 stage amp fed directly into the 567. According to my scope, it did amplify the 40khz signal. Maybe the ones I got are much better than spec.

It does detect the signal loop wire, but since I shrunk the board, it now also detects metal objects. I think it is detecting the internal signal in the 567.

If I use one of the op-amps you mention with reasonable gain and better power decoupling on the 567, should I do anything else for filtering, etc.

What is the purpose of R3 and C4 in the schematic I posted?
Now I have them directly connected, could that also be part of my problem?

Should the 2 leads of the pickup coil attach to the input of the op-amp like the other circuit here with the transducer?

Thanks again for your help. I am way out of my normal realm here.

Craig

 

[audioguru]

That is not my circuit, I found it online and it never worked.

Maybe it originally used an odd LM3900 opamp which is obsolete and sombody tried an ordinary LM348 opamp instead.

I ended up using the 358 as a 2 stage amp fed directly into the 567. According to my scope, it did amplify the 40khz signal. Maybe the ones I got are much better than spec.

The datasheet shows that its max output is typically only 3V p-p at 40kHz and its max gain is only about 25.
But the NE567 is pretty sensistive if it is connected properly.

If I use one of the op-amps you mention with reasonable gain and better power decoupling on the 567, should I do anything else for filtering, etc.

Attach the schematic of your 2-stage amplifier.

What is the purpose of R3 and C4 in the schematic I posted?
Now I have them directly connected, could that also be part of my problem?

R3 is not needed and a direct connection between the opamp and the DC blocking capacitor is fine.
C4 is needed. All circuit examples in the datasheet of the NE567 show an input DC blocking capacitor.

Should the 2 leads of the pickup coil attach to the input of the op-amp like the other circuit here with the transducer?

It depends on your circuit. Probably not because a piezo transducer does not conduct. Your coil is a DC dead short.

 

[AVR_Craig]

I didn't have a schematic for it. I did the PCB layout directly off the circuit on the breadboard. I was sketching it out to scan and post when I found a missing trace on the board. It is in the gerber file, but never made it to the board.

Anyway, it works now, as well as it did on breadboard. I do see the 358 clipping the signal at around 3 volts. I will order some MC34072 and a good selection of resistors and caps and see if I can improve it with some further research.

What value and type should I use for C4, the DC blocking cap between the op-amp and 567? Is there a formula for this based on freq? The datasheet shows it on every example, but offers no help on selecting it.

Thanks again for all your help. I am satisfied with the performance for now, it should only get better with the faster op-amp. Once I have this project done and I get more time, I will revisit this so I can actually understand all this better.
For now it is back to the land of 1's and 0's where everything is so much simpler.

Again, thanks.
Craig

 

[audioguru]

The lousy old LM358 limits its max output at 40kHz to only 3V because its output becomes a triangle waveform. It can't swing quick enough to go any higher before it must change direction.

The calculation for the blocking cap that feeds the input of the NE567 is calculated with the frequency where the response is reduced 3dB and the input resistance which is about 20k ohms (in the dataheet). To avoid loss then make the calculation at a frequeny that is divided by 5 (8kHz). So a 0.001uF (1nF) coupling capacitor would cause 8kHz to be down 3dB but 40kHz would be almost flat with hardly any loss. C= 1 divided by (2 pi fR).

Your 0.47uF capacitor was 470 times bigger than necessary.

 

[AVR_Craig]

The lousy old LM358 limits its max output at 40kHz to only 3V because its output becomes a triangle waveform. It can't swing quick enough to go any higher before it must change direction.

My scope shows this exactly.

The calculation for the blocking cap that feeds the input of the NE567 is calculated with the frequency where the response is reduced 3dB and the input resistance which is about 20k ohms (in the dataheet). To avoid loss then make the calculation at a frequeny that is divided by 5 (8kHz). So a 0.001uF (1nF) coupling capacitor would cause 8kHz to be down 3dB but 40kHz would be almost flat with hardly any loss. C= 1 divided by (2 pi fR).

Reading that makes me painfully aware of how little I know about analog electronics.
Thanks again for your help, I will post my results when I get it all straightened out.

 

[Lesh]

The LM358 has a GBP / Unity gain bandwidth of 700khz, so configured with a gain of 100 it should do 7khz. Configured with a gain of 10 it should to 70khz. With a gain of 1000 it will only do 700hz. A lot depends on the circuit doesn't it?

Edited to add, good point on the slew rate.

Edited again to add: A wise man said, "In theory, theory and practice are the same. In practice, they are not."

 

[audioguru]

The LM358 has a GBP / Unity gain bandwidth of 700khz, so configured with a gain of 100 it should do 7khz. Configured with a gain of 10 it should to 70khz. With a gain of 1000 it will only do 700hz.

Nope.
The bottom frequency scale is logarithmic so half a division is 0.316 times and not 0.5 times.
Therefore the GBP is 400kHz when its supply voltage is only from 10V to 15V as shown on a graph on its datasheet:
1) With a gain of 100 (40db) its response drops above about 4kHz.
2) With a gain of 10 (20dB) its response drops above about 40khz.
3) With a gain of 1000 (60dB) its response drops above about 400Hz.

The poor slew rate is shown on another graph on its datasheet. Its output is 13.5V p-p up to only 5kHz. At 40kHz its max output is only 3.0v p-p.

The better TL07x or MC3407x opamps have a gain of about 100 at 40kHz and their outputs have a max output of 27V at up to 100kHz.

 

[Lesh]

Nope.
The bottom frequency scale is logarithmic so half a division is 0.316 times and not 0.5 times.
Therefore the GBP is 400kHz when its supply voltage is only from 10V to 15V as shown on a graph on its datasheet:
1) With a gain of 100 (40db) its response drops above about 4kHz.
2) With a gain of 10 (20dB) its response drops above about 40khz.
3) With a gain of 1000 (60dB) its response drops above about 400Hz.

The poor slew rate is shown on another graph on its datasheet. Its output is 13.5V p-p up to only 5kHz. At 40kHz its max output is only 3.0v p-p.

The better TL07x or MC3407x opamps have a gain of about 100 at 40kHz and their outputs have a max output of 27V at up to 100kHz.

Which graph? I think we are looking at different data sheets. TI and ST data sheets list a spec of 700khz min. ST does have a GBP to temperature graph.
Which manufactures data sheet are you looking at?

 

[audioguru]

Which graph? I think we are looking at different data sheets. TI and ST data sheets list a spec of 700khz min. ST does have a GBP to temperature graph.
Which manufactures data sheet are you looking at?

National Semi invented the LM358 so I look at their datasheet.

ST Micro says that its GBP is 1.1MHz when it has a 5V supply, but its graph shows 1.1MHz with a 30V supply and only about 500KHz with a 10V to 15V supply. ST Micro frequently get things mixed up.

TI says its GBP is only 700kHz typical when it has a 30V supply. No graphs.

 

[tippa]

i too want an receiver circuit for amplifying 40khz signal from GL402R receiver... i want amplified signal of max 5v what should be the gain and can i use LM358 for that...plz anyone provide me the circuit for that

 

[audioguru]

At 40kHz an LM358 has a max output level of only 3V p-p and a max gain of only 25 if the supply is between 10V and 15V. If the supply is 5V then it can hardly amplify 40kHz.

An MC34071 single opamp, MC34072 dual opamp or MC34074 quad opamp) also have a minimum supply voltage of 3V and have inputs that work at ground in a single supply circuit but have full output level up to 100kHz and have a max gain at 40kHz of 200.