Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Help with Peak Detector using LM324 for 50Hz AC (One LM324 works, one does not)

Status
Not open for further replies.
S

ste2006

Advanced Member level 4
Joined
May 1, 2012
Messages
118
Helped
3
Reputation
6
Reaction score
3
Trophy points
1,298
Activity points
2,226
Hi,

I have designed and am using a peak detector for a 50Hz AC Signal coming from a Current Transformer and ultimately into a microcontroller ADC, Problem is i did the circuit up on bread board with a TH LM324N and the circuit worked perfectly,

Now i have gone and got a PCB made up using a SOIC LM324DG4 and while the peak detector works up to about 0.6V once the input goes above this the output just saturates to 5V

I have a schematic below but it is rather simple

Peak Detector.jpg

The two OP-Amps i purchased in RS in case anyone can see a difference,

TH one that works 268-0002
SOIC that saturates 661-0521

I ultimately need it to work to about 4V P-P

Anyone any ideas it would be great as this is really puzzling me,

Thanks,
 

Now i have gone and got a PCB made up using a SOIC LM324DG4 and while the peak detector works up to about 0.6V once the input goes above this the output just saturates to 5V
Click to expand...

The strange thing with this observation: According to datasheet (and my personal experience), LM324 positive output voltage doesn't exceed Vcc-1.2V (about 3.8 V). This suggests a partly different circuit, or the connected processor is forcing the respective pin.

A possible problem may be brought up by driving the LM324 input below the supply rails, which is violating the maximum ratings and can cause unexpected amplifier behaviour.
 

No Circuit is definitely identical, It is the chip, I went and soldered the SOIC LM324 on to a piece of scrap board and added it in place on the breadboard and exact same results, Saturates above about .6V

I dont mind swapping to a different Op Amp with the same package but it would be nice to know what i am looking for as i will no doubt damage the board by pulling on and off different IC's

I also have taken off the processor as i thought that may be a problem but still same result, I also removed the resistor R10 and problem still exists so it is obviously a problem with the first rectifier stage of the circuit,

I tried different IC's (same type) in case it was a bad IC but no same result,
 

ste2006 said:
...while the peak detector works up to about 0.6V once the input goes above this the output just saturates to 5V...
Click to expand...
Judging by the simplified schematic in the LM324 datasheet, it looks like the output may go high when the non-inverting input goes down to about one diode drop below the negative supply rail.

You can prevent that by adding a simple clamp circuit as shown below. Ideally the diodes should be chosen so that D2 has a higher forward voltage drop than D3, otherwise there will be some inaccuracy with small input voltages (output will be slightly too high).

 

Ok think i have solved it by changing R9 & R11 up to 100K

Seems to work but not quite sure why???
 

The larger resistor effect is pretty plausible, because you need to drive a certain current below ground to make the part act irregular. And the effect can be different with different chip versions.

My preferred solution is a schottky clamp diode between non-inverting input and ground. The solution by godfreyl works as well, bus is clamping the input at a slightly positive level.
 

Ok thanks guys, Problem is the PCB is designed now and manufactured and they are not that cheap to get done, Changing a resistor value is easy, adding a clamp into the system involves a new PCB thats all,

Anyone see any strange issues occuring with the larger resistors??

Thanks,
 

Not all LM324 are created equal when it comes to driving source or sink current from a 5V supply. You might get 5mA on some and only 1~2mA on others. They like to rate them at 15V or more because their drive current is higher. You may want to add a small parallel cap on the input to limit noise bandwidth.

This may explain why higher resistor values were needed.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top