TI LMV358 Op-Amp. No notch or dot.

[Boltar]

Hi,

This is a TI LMV 358 ID SOIC opamp

I can't see any notch or dot on the package anywhere, top or bottom, I've assumed pin 1 was the bottom left pin, but I think I might be wrong as my circuit isn't working, but I'm not 100%, I may have made a mistake elsewhere, but I'd be happier knowing for sure which way round this thing goes before I go any further.
This is the datasheet: http://www.farnell.com/datasheets/1836422.pdf

Thanks for any help.
Cheers

[Precipice]

The line is at the pin 1 end, so your assumption is correct (Text also tends to be the right way up with the pin 1 at bottom left on SOP packages. Not necessarily so on QFx packages, though. )

[Boltar]

Thanks.

 I was hoping I'd put it in the wrong way round as that would have been an easy fix. Means it's something more complex Ah well at least I know now, thanks a lot.

[Boltar]

This is the circuit I'm having problems with.


ADPWR = 5V supplied from a microcontroller IO pin when required.
ASENSE = ADC pin on microcontroller (5V reference)
ATM = A load on the ground rail.

The idea is that when the resistance of ATM is required, the microcontroller brings ADPWR high which powers the opamp and also sets up a voltage divider between R12 and R13+ATM (R13 is just to make sure some voltage goes into the non-inverting input even if ATM is a short circuit. The op amp is in a simple negative feedback amplifier configuration to multiply the relatively low voltage (usually about 27-28 times), the pot is used to calibrate the upper limit of the detection (higher upper limit = less accurate but more range, lower upper limit = more accurate but less range). D3 is required to prevent damage to the microcontroller as another circuit also sometimes applies current to ATM (not at the same time the resistance is being measured). The reading are slightly off linear (they show a small parabolic curve), but 2 lines of code can compensate for that. I'm getting an ADC reading of 0 all the time. Can't figure it out.


I've used this circuit before in other projects (without terminating the unused gate) and it's worked fine, so I'm a bit lost. I'm going over my SMD soldering atm as I'm very new to SMD stuff. I'm not sure if I've terminated that gate correctly, it was a bit of a lazy way.

EDIT: Something's definitely wrong with the op-amp. I've desoldered it and put a multimeter across its power input pins. It was reading about 2 ohms. When I tried the same with a new, never used one I get 3.5 megaohms.

EDIT2: Hmmmm, I just sacrificed a new one to test a theory. All I did was apply 5V to pin 8 and ground to pin 4 assuming pin 8 is the top left pin in the picture and pin 4 is the bottom right pin. Then I multi metered it afterwards, it now reads 2 ohms across the VCC and ground pins. Buggered it just by correctly applying power.

EDIT3: No sorry, I messed that up. What I think is happening is that when ATM is powered up properly (this is from a 12V supply) the 12V is going into the non inverting inputs of the op amp, I assumed that this wouldn't matter as the op amp is powered down when this happens (both supply pins are grounded) and also I thought the inputs of an op-amp are at some crazily high resistance so it wouldn't matter, but it definitely seems to be damaging the device. I might be able to put a transistor or mosfet between ATM and the NII, would this be a valid solution?

[GK]

If you rotate any DIP, SOIC, TSOP, etc., so that the text is the right way up, pin 1 will be at the bottom row left. Simples.

[janekm]

You might want to read up about latchup. Applying power to input/output pins while VCC==GND is definitely not a good idea. (Or watch eevblog #16 )

[amyk]

If you rotate any DIP, SOIC, TSOP, etc., so that the text is the right way up, pin 1 will be at the bottom row left. Simples.

I've come across a few exceptions to this (remarked fakes? they still worked though...), so it's not 100% conclusive but quite close.

As for OP's IC, there are two indicators: a thick line on the left and a beveled side.

[Tac Eht Xilef]

If you rotate any DIP, SOIC, TSOP, etc., so that the text is the right way up, pin 1 will be at the bottom row left. Simples.

Apart from the innumerable exceptions...

The SOIC package shown has a chamfer along one side. If you rotate the chip so the chamfer is on the left, pin 1 is top left. All as per the linked datasheet.

OP, you were looking at the wrong package drawing - DGK (VSSOP) vs D (SOIC). Easy mistake to make...

[janekm]

actually re-reading your post, I think you are saying that you are applying 12V at ATM? In which case 12V will flow through the protection diodes on the I/O pins into the internal VCC on the chip (have a look at the simplified schematic in the datasheet).

[GK]

If you rotate any DIP, SOIC, TSOP, etc., so that the text is the right way up, pin 1 will be at the bottom row left. Simples.

I've come across a few exceptions to this (remarked fakes? they still worked though...), so it's not 100% conclusive but quite close.

I'd probably vote for remarked fakes. I've never personally come across an exception. Have soldered in SOICs of at least 1000 different part numbers now and have yet to blow one up (for reasons of orientation that is)!

[Galaxyrise]

I thought the inputs of an op-amp are at some crazily high resistance so it wouldn't matter

Applying 12V to the input exceeds the valid input range of the device: both the absolute input voltage (which is really bad to do) and common mode input range, so all bets are off.  More datasheet parameters for you to pay attention to

There are opamps that can handle inputs at (or even over) their supply voltage, like the LT1637.  "The LT1637 has a unique input stage that operates and remains high impedance when above the positive supply."  If you still want the opamp to be off when not taking a measurement,  you could drive the shutdown pin with your mcu rather than driving the supply pin. 

[true]

If you rotate any DIP, SOIC, TSOP, etc., so that the text is the right way up, pin 1 will be at the bottom row left. Simples.
...
I've never personally come across an exception

PIC32 in DIP.

[Boltar]

Applying 12V to the input exceeds the valid input range of the device: both the absolute input voltage (which is really bad to do) and common mode input range, so all bets are off.  More datasheet parameters for you to pay attention to

Yes I'm sure this is the reason. When I've done this before it's been with bigger through hole op-amps that can handle it, these little ones can't. It's all part of the learning process innit?
I'm going to try removing D3 completely and replacing with a wire bridge. Replacing R13 with the collector and emitter of an NPN transistor and the base of this transistor via a suitable resistor to ADPWR. If I'm right, the transistor will turn on when ADPWR is HIGH allowing current to pass through it to ATM. When ADPWR is off (0V IO pin is LOW) and 12V is applied to ATM, the current should stop at the transistor emitter because it's basically a diode. On the next design I'll try using an op-amp like the one you suggested.

The 12V and switch is a simplification.


Thanks for all the help guys.

[BravoV]

A picture worth thousands words ...

[Boltar]

Cool, yes now I see it. So I guessed right with pin 1 initially, but now I know for sure . Now just to see if I can stop 12V getting into the inputs.

EDIT: Just in case anyone is interested. I replaced the burned out opamp with a new one and bodged in the transistor arrangement i mentioned above (using a TO92 2N3904 and 1K resistor on the base) . All seems to be working fine now.  I can fire ATM and the op-amp seems to survive. I'm getting readings back from ASENSE . The transistor seems to have mucked up the simple parabolic deflection though, it now seems to have the same parabolic curve but the overall direction of the line has changed from 45 degrees to something like 30 degrees, weird, but I can deal with that in code. I Just have to make absolutely sure I never fire ATM and have ADPWR HIGH at any time.


Not pretty but it works, needs a good going over with IPA too.

Cheers guys, I'm a happy chappy.

[T3sl4co1l]

If you rotate any DIP, SOIC, TSOP, etc., so that the text is the right way up, pin 1 will be at the bottom row left. Simples.

I've come across a few exceptions to this (remarked fakes? they still worked though...), so it's not 100% conclusive but quite close.

As for OP's IC, there are two indicators: a thick line on the left and a beveled side.

The worst I've ever seen was a 7406 (not SN7406 or anything, no prefix) manufactured in the 4th week of 1974 ("7404"), written sideways on the ceramic package, no other info (I forget if the dot / semicircle / whatever was present or legible).

Tim