I need opamp suggestions

Ian Rogers

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• May 3, 2017

• #1

I was thinking along the lines of an LMH7623 only because I've used it before... BUT!! Its a video amp so might not be suitable

The diode ( D1) is also a bit screwballed... So I need suggestions for that as well..

The system has an optocoupler and chopper wheel, the wheel has about 28 cuts and can spin around 10 times a second, so reletively slow frequency..



As you see U2 is a bit sad....

 

tomizett

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• May 3, 2017

• #2

Is U2 the (suspected) opamp - it looks like it's only got 6 pins. If that's the case then it's a bit unusual.

 

Ian Rogers

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• May 3, 2017

• #3

tomizett said:

Is U2 the (suspected) opamp - it looks like it's only got 6 pins. If that's the case then it's a bit unusual.

Click to expand...

Yeah... Very unusual.... It has 5 pins.. It's a sot-23....

 

ronsimpson

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• May 3, 2017

• #4

If this link works:
https://www.digikey.com/products/en...fiers-instrumentation-op-amps-buffer-amps/687
Scroll to most right column, pick a package like SOT-23-5, and "apply filter".
I got 1000 different choices.
MicroChip has the 5 and 6 pin opamps in several different pinouts. There are too many choices as to which pins are power and ground. Watch out for that!

 

Ian Rogers

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• May 3, 2017

• #5

The chip didn't blow... The chopper wheel became loose and crashed into it!!

The LMH6723 is a 370Mhz amp... I'm sure it'll be okay!!!

 

Ian Rogers

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• May 4, 2017

• #6

Okay... Didn't work... The opamp fitted was the LMH6723... but now I seem to have a bit of an offset... Here's the circuit



It would appear now that the output NPN is on all the time... What simple change would help switch off the NPN... I have a good output at the opamp but it is higher than it should be...

 

AnalogKid

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• May 4, 2017

• #7

Are you sure about that circuit? 103K seems a bit large for a base resistor, and very large for a current feedback opamp. Plus, what is with the two 10K's?

ak

 

jpanhalt

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• May 4, 2017

• #8

LMH6609 (video, 900 MHz), AD8055 (300 MHz, +/-5V) and AD8033 (80 MHz, +/- 2.5V to +/-12V). I have used all three and mostly the latter two. All are available in SOT23-5 with same pinout. The AD8033 has a wider supply range and FET input.

John

 

Last edited: May 4, 2017

Ian Rogers

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• May 4, 2017

• #9

Hi AK..

The circuit is above ( photo )... The SMT are 1003 ( 100k) and two 1002 ( 10k ) unless I've gotit wrong... The 1003 is connected to the - input and the 3301 ( off screen 3k3..)

 

jpanhalt

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• May 4, 2017

• #10

The LMH6723 is a CFB op-amp (https://www.ti.com/lit/ds/symlink/lmh6723.pdf ). Do you know what the original was? Most standard CFB circuits I have seen have lower valued feedback resistors. I just assumed the original op-amp was VFB, as are the 3 options I posted.

John

 

Ian Rogers

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• May 4, 2017

• #11

So rip out the 100k and put in a 10k?? If you look at the photo, the chopper wheel destroyed the top of the opamp..

The new one is working as an op amp, but the small voltage when off is turning on the NPN... Sorry it seems like I ignored you John but I had to get something ordered... I took the risk as I use these to upgrade the speccy's composite video out...

 

Ian Rogers

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• May 4, 2017

• #12

Something just hit me on the head!!! The pinout may be different ( As Ron pointed out )

If IN- and OUT are reversed it would make a lot more sense.... The two 10k make up a voltage divider with the 100k being the gain resistor ....

AARRGGHH!!



Two bluddy types..... I must need a type 2..

 

jpanhalt

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• May 4, 2017

• #13

CFB op-amps usually have a manufacturer's recommendation for the feedback resistor. The ones I have used have called for resistors of 1k to 500R. Another difference is the significant difference in input impedance for the inverting and non-inverting inputs of CFB. I tried to find a datasheet for your original op-amp and couldn't. But its circuit certainly looks like a VFB op-amp to me.

This tomb, Chapter 8 describes the differences: https://www.cypress.com/file/65366/download

I would just get a 12V tolerant op-amp, like the AD8033, and see what happens. A CMOS version may also work because it will probably have less offset.

John

 

Ian Rogers

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• May 4, 2017

• #14

LM321MF... Seems to be the selection I need

Ordered 5.... 3 to 32v operation 1Mhz... Its only a opto input!!

 

jpanhalt

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• May 4, 2017

• #15

I read your post after I posted. Obviously, my recommendations are type 1. Sorry.

John

 

Ian Rogers

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• May 4, 2017

• #16

AD8033 is the wrong pinout John....

 

Ian Rogers

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• May 4, 2017

• #17

Anyway... Thanks for the help....

Actually... It was AK's remark that got me thinking... The OPAMP circuit did look weird!!!!

Cheers everyone..

 

Mikebits

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• May 4, 2017

• #18

Ian Rogers said:

Something just hit me on the head!!! The pinout may be different ( As Ron pointed out )

If IN- and OUT are reversed it would make a lot more sense.... The two 10k make up a voltage divider with the 100k being the gain resistor ....

AARRGGHH!!

View attachment 105888

Two bluddy types..... I must need a type 2..

Click to expand...

Whew, I was looking at your earlier drawing and thought I had to restudy op amp theory. The voltage divider on the amp output really had me perplexed. Glad you made your discovery for my sanity

 

Ian Rogers

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• May 4, 2017

• #19

The bit that gets to me... Every 8 pin and 14 pin dip follows a standard... I must have looked at 100's of SOT23-5 pinouts. I never saw type1 or type2... That's why I looked for an earlier type...

 

Ian Rogers

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• May 8, 2017

• #20

LM321MF... Worked like a dream....... There was a really small resistor sitting on the output NPN... This was also gone... I have no idea what value it used to be, but as its really only a fuse, I used a 0R47... Seemed to do the job...