OpAmp Test Jig

[Samuel Groner]

Hi

I'm planning to do some serious opamp tests next year and I need a good test jig for that. All tests should be doable with a good signal generator, 100 MHz scope, a DVM and an AP System One. They are based on Jung's writing but include several modifications and additions from my side.

The sockets are laid out for the 2520 footprint, IC's get a board with a pair of local 100 nF decoupling caps.

Test 1
[removed]
For calibration, RV101 is set (with S101 open) to give overall 60 dB gain. Bandwith limiting to 20 kHz is done with the AP System One (or an external filter).

TP101/102 is used to set desired supply voltage. S102 allows trimming of the closed-loop frequency response of two-pole compensated opamps (they might have a bump in the response without a phase lag capacitor).

S101 closed gives voltage noise measurement and S101 open current noise measurement.

Expected precision: better than 5% down to 0.6 nV/sqrt(Hz) and 100 fA/sqrt(Hz).

Test 2
[removed]
Small- and large-signal transient response, slew-rate and THD+N in inverting unity-gain configuration.

Test 3
[removed]
With S301/302/303 open: small- and large-signal transient response, slew-rate and THD+N in noninverting unity-gain configuration.

Short-circuit protection with S303 closed.

With S301/302 closed: small- and large-signal transient response with capacitive loads.

Test 4
[removed]
With S401/402 open: THD+N for general transfer linearity.

With S401/402 closed: THD+N for output linearity (1k8, 600 and 150 ohm load).

Test 5
[removed]
THD+N for input linearity.

Test 6
[removed]
With S601/602 closed: input offset.

With S601 and/or S602 open: input bias and input offset current.

Expected precision: better than 10% down to 50 uV and 50 pA.

Now a few questions:
* Jung gives a precision of 0.1% for R503; this must be a typo, no?
* are the expected measurement precisions realistic?
* is the polarity of C106 right to avoid reverse bias?
* I plan to use DIL-switches; are these robust enough to accomodate the rather hefty currents in the short-circuit and output linearity test?
* would you add a switch to disconnect "In 1" to avoid RF pickup when doing current noise measurements?
* any other drawing/conceptual error or suggestion?

Thanks for your input and help!

Samuel

 

[PRR]

* Jung gives a precision of 0.1% for R503; this must be a typo, no?

10Ω 0.1% is a standard test-lab part, and Jung probably has a box-full at work. I don't see how 10% here would really foil your measurements. Don't use anything inductive: 5% carbon-film is probably fine.

* are the expected measurement precisions realistic?

Getting noise that low will need experience or a lot of try-this try-that.

* would you add a switch to disconnect "In 1" to avoid RF pickup when doing current noise measurements?

Same answer; try and see.

* is the polarity of C106 right to avoid reverse bias?

If I read it right: voltage on C106 should be zero and will always be less than a volt, which is fine, especially in short-term tests. Would not hurt to put a spare DC voltmeter in front of C106: if it shows much DC, somethin is wrong.

* I plan to use DIL-switches; are these robust enough to accomodate the rather hefty currents in the short-circuit and output linearity test?

They should be fine for testing. I would not take them into orbit and bet my life that they would fire the retro-rockets to bring me back.

 

[tk@halmi]

Wow, this is good stuff! I have a very simple test jig, but it is nothing like what you are proposing here.
Do you have a good reference paper/source for characterizing opamps?

Thanks,
Tamas

 

[bcarso]

The DIP switches are convenient and small, thus low stray C. Other than that, in my experience they are unreliable with much use. If you are going to be switching a lot I would substitute those square pin headers and buy a bunch of the little slip-on shorting jumpers. The jumpers will wear out but you can change them, and the pins will last a long time.

Ditto as far as PRR's comments, on measuring that low a noise density. If your fixture noise is at least very stable you can do a fairly good root-sum-of-squares accounting of it, even if it exceeds the noise of the device under test.

 

[Samuel Groner]

Thanks for your answers and comments.

Getting noise that low will need experience or a lot of try-this try-that.

OK, point taken.

Voltage on C106 should be zero and will always be less than a volt, which is fine, especially in short-term tests.

I should have written my question more clearly: yes the voltage should be close to zero but there is a systematic offset due to the input current and DC source impedance imbalance--I guess it doesn't hurt to put the cap the right way around even if the voltage is too small for true reverse bias. Is my thinking right that this offset has negative polarity? I usually get confused when trying to figure out that polarity... :roll:

Do you have a good reference paper/source for characterizing opamps?

Most ideas are taken from "Audio IC Op-Amp Applications" by W. Jung, chapter 2.

Web_Ch1_final_R.pdf has some more info on tests for less audio related parameters.

If you are going to be switching a lot I would substitute those square pin headers and buy a bunch of the little slip-on shorting jumpers.

Good idea--I guess these are cheaper and smaller as well.

Samuel

 

[Samuel Groner]

Oh, I messed up test 5 in the first revision--it should of course be different from test 4 and expose the opamp input with full CM voltage. The new revision includes these and a few cosmetic changes:
[removed]

Just to make sure: this didn't change anything about the required precision of the 10 ohm resistor (now R502), right?

VCC/VEE is supplied by a lab PSU and between +/-15 V and +/-35 V.

Samuel

 

[CJ]

We built this DIY test jig back when the 741 was still "da bomb".
They were cheap, but needed to be screened for the occasional stinker as far as dc offset. Just some machined teflon and a control box. Probably not needed in your case unless you need to check quanties at a time.

The chips get "plungered" into the homemade test socket. You can see one in there now:

[/list]

 

[Samuel Groner]

He he, cool stuff! No, no need for quantity testing from my side though!

Samuel

 

[Samuel Groner]

Here's the final schematic and the PCB design:
OpAmp_Test_Jig_r1.pdf
OpAmp_Test_Jig_PCB.pdf (0.5 MB)

Samuel

 

[tk@halmi]

Samual,

Thank You for sharing this. I am sure it took a lot of time compiling information to arrive at your solutions. Many folks don't realize how difficult it is to obtain meaningful measurements with the simple equipment most of us can afford, and information on testing is often poor. And that these measurements are important steps to prove a design that may or may not lead to a working product. Yes, the ear is the final judge, but all the steps that preceed the first listening test are often the overwhelming majority of the journey.

Cheers,
Tamas

 

[Samuel Groner]

My pleasure!

So far I decided to test the following IC's:
Single:
* AD797AN (Analog Devices)
* NE5534AN (Texas Instruments)
* OPA604AP (Texas Instruments)
* OPA627AP (Texas Instruments)
* OPA134PA (Texas Instruments)

Dual:
* OP271GP (Analog Devices)
* OP275GP (Analog Devices)
* AD818AN (Analog Devices)
* AD828AN (Analog Devices)
* NE5532AN (Signetics)
* NE5532AN (Texas Instruments)
* OPA2604AP (Texas Instruments)
* OPA2134PA (Texas Instruments)
* LM833N (National Semiconductor)
* MC33078P (ON Semiconductor)

For me an audio opamp should at least meet the following specs:
* voltage noise <= 10 nV/sqrt(Hz)
* slew-rat >= 5 V/us
* GBW >= 5 MHz

In addition to this it needs to be:
* a voltage-feedback design
* operated at +/- 15 V or higher
* unity gain stable
* DIP-/DIL-8 format

Any other suggestion fitting that list?

[Edit:] I forgot the ones from LT on my list--will add them ASAP...

Samuel

 

[clintrubber]

Nice initiative !

Just wondering since you was using the 2520 footprint: no DOA's in your list ?
Those might not meet the stated specs, but hey, the 5534 doesn't meet another stated requirement either :wink: (but that's easily solved by a comp-cap)

Any other suggestion fitting that list?

Maybe adding some non-candidates to the list to see why they're indeed better at something else than audio ? Say a DC-precision opamp.
Or 4558 :shock:

And maybe some comparing with http://www.dself.dsl.pipex.com/ampins/webbop/opamp.htm ?
I'm fairly sure you'd make some people here very happy if you managed to find a few errors on that page :wink:

But serious, some checking to see if it's essentially in line with any other thrustworthy result out there would be of use of course.

 

[Samuel Groner]

No DOA's in your list?

Not in my list, but in my mind, of course.

The 5534 doesn't meet another stated requirement either.

External compensation is OK.

And maybe some comparing with www.dself.dsl.pipex.com/ampins/webbop/opamp.htm ?

I'm looking forward to compare my findings with these. In fact that page is one of the main reasons I started looking into some more detailed tests.

Maybe adding some non-candidates to the list to see why they're indeed better at something else than audio?

Would be interesting. However my intention is basically to find good (for my taste) audio IC's, not to proof the world that some are not. If I have some time left I'll throw in a few non-candidates but I very much doubt there is considering the upcoming Tonmeister studies :sad: (just because of the missing time, actually :grin: ).

Samuel

 

[clintrubber]

[quote author="Samuel Groner"]

No DOA's in your list?

Not in my list, but in my mind, of course.
[/quote]Nice.

The 5534 doesn't meet another stated requirement either.

External compensation is OK.

Was just kidding of course.

And maybe some comparing with www.dself.dsl.pipex.com/ampins/webbop/opamp.htm ?

I'm looking forward to compare my findings with these. In fact that page is one of the main reasons I started looking into some more detailed tests.

Niced overview indeed ! And 'refreshing' to see that those various claims for audio-excellence of newer opamps often don't surpass the 553X.

Maybe adding some non-candidates to the list to see why they're indeed better at something else than audio?

Would be interesting. However my intention is basically to find good (for my taste) audio IC's, not to proof the world that some are not. If I have some time left I'll throw in a few non-candidates but I very much doubt there is considering the upcoming Tonmeister studies :sad: (just because of the missing time, actually :grin: ).

You're right, first things first. Tonmeister, I like that name. :thumb:

Bye,

Peter

 

[Samuel Groner]

After sleeping over it I realised that the output buffer for the noise test needs to be complementary and beefier to accomodate opamps with large offsets (up to about 100 mV). Schematic and layout are update.

Here the new IC test list:
Analog Devices:
AD797ANZ (single)
AD829JNZ (single)
OP271GPZ (dual)
OP275GP (dual)
OP467GPZ (quad)
OP471GPZ (quad)

Texas Instruments:
NE5534AN (single)
OPA134PA (single)
OPA604AP (single)
OPA627AP (single)
MC33078P (dual)
NE5532AN (dual)
OPA2134PA (dual)
OPA2604AP (dual)
TL4581PE4 (dual)
RC4580IP (dual)

Linear Technology:
LT1128ACN8 (single)
LT1363CN8 (single)
LT1468CN8 (single)
LT1213ACN8 (dual)
LT1469CN8 (dual)
LT1630CN8 (dual)

National Semiconductor:
LM833N (dual)

Signetics:
NE5532AN (dual)

More suggestions still welcome!

Samuel

 

[Bauman]

Nice stuff Samuel!! Pls test OPA637AP if you have a chance!!!

Exciting to hear the results!!! I see you´re planning some DOA compatible footprint too!!! :green:

cheers!
Fabio

 

[Samuel Groner]

OPA637AP

I decided to include unity-gain stable amplifiers only for this test series as it seems to be difficult to derive a procedure which allows fair comparison for different compensation levels. In addition to this, the required PCB would need to be far more complex to accomodate the various switches etc.

Perhaps later when I have more experience I'll do another run...

For the OPA637 I guess we can derive the results from the OPA627. The OPA637 will have similar output stage and basic transfer linearity if used at 5x higher closed loop gain. Slew-rate related distortion will be much lower due to the inherent higher slew-rate. CM input linearity will be comparable but is less important as we cannot run it at low noninverting configurations without tricks.

Samuel

 

[Bauman]

Cool Samuel, I suggested this cos last time I tried both 627 and 637, 637 was more interesting sounding...

cheers!
Fabio

 

[khstudio]

What ever happened with your tests... I'm interested :green:

I'd love to be able have a jig to test my op-amps before installing them... even for a few standard things like Noise/distortion, crosstalk & balance (between the 2 amp in a DUAL of course)

And since my mixing console is loaded with JRC 5532D's I'd like them added to the list.

I started a 5532 thread a while back & felt the JRC's had more PUNCH & Character than all the other brands.

The ones in my board are REAL JRC's not NJR's - New Japan Radio.
Although I've tried these recently & they are cleaner but not as full sounding... can't say why.

 

[Samuel Groner]

Well, I'm waiting for a free week to run the tests... The jig is tested, I got samples for most opamps already and have worked out the test details but will need to write some Matlab code for the planned FFT and time domain analysis.

Samuel