TIA-PD compensation, by eye

It depends on the frequency. An unterminated cable at 1/4 wavelength reflects back as a short.

For 7.5 ft coax, 7.5*12*2/3 = 60 ns.

This is in the ballpark of what he is seeing. However, the main problem is trying to do a 200 MHz op amp on a two-sided pcb with no ground plane.

That's not a photodiode, it's a solar panel.

What's its capacitance?

Right, It's cool (to me) that 50 ohms on the end cures it. If this ever turns into a product I'll pick a slower opamp, or add a line driver. I've got some opa827's from the same order. slower, less noise.

George H.

Joerg! I'm not sure why you are interested in this (minor) project, but I love it! You know it's cool that this pcb almost works with an opamp that's ~50 times faster than the opa124A it was designed for.

Grin, I could stick a Big damping cap across the low end, For this device the only app for low gain is as a power meter, DC. (at least at the moment)

Hmm well I keep talking about the stray feed back capacitance, which at the moment is the big problem. I'm not sure a ground plane will solve this problem, but I'll give it a try. (I was thinking more of a driven shield at the minus input, but except for high frequency that's almost the same as ground.)

So I took out a blank pcb today and first measured the trace C. ( ~1" bus wires into production's SRS RCL meter, 1.7 pF, it's not all that accurate at the pF level. I built up the circuit with a 1 Meg R lying flat from the switch wiper hole, f_3dB = 150kHz, ~1pF. I then soldered bus wire from the wiper and bent the TH resistor into a hairpin, (see pic above) f= 120 kHz. I've always wondered what the C of a hair pin 1/4 W TH R was, about 0.3pF more than lying flat, is my answer.

I took out the bus wire and added the switch f~ 70 kHz, ~2.3 pF, the switch added about 1 pF. Adding in 9 more hair pin R's brought f_3dB down to

Tomorrow I'll try some 0805's.

Hmm I need ~1M (or 10M) ohm because of low currents. Not much light, for some apps that's because of a weak source, for other atomic physics stuff it's because light saturates the transitions, (more light, less signal) For some stuff you have to turn off the room lights, or hang black cloth's here and there. you can do lenses and stuff,but for beginners, that's complicated.

George H.

Grin ~120pF at 10V. (according to the data sheet) But it's the stray feedback C of ~5pF that is my problem, (at least that's where I am.)

George H.

es]:

That formula's wrong--propagation velocity is about 2nS/ft on FR-4, not 2/3rds of a nS per inch!

'c' in vacuum is 3x10^8 m/s, and about 2/3rds that in coax, right?

A two-way trip through 7.5' of coax is roughly 5 meters, so 5m / (2/3 * 3x10^8 m/s) = 25nS.

George's ringing is about 62-ish nS, too slow for the cable to be resonant.

Cheers, James Arthur

Yes, we agree on that. Ye olde small resistor Rs in series with the output, and the feed-forward feedback cap he already has fixes that. Cf .------||----. | | | |\ | >--+--|-\ | | >------+--[Rs]-----> to coax

Cheers, James Arthur

.--------[Rfb]-------. | Cf |

I sketched in Rfb, in case that wasn't clear.

There's still the problem of that solar panel loading the summing junction, but this fixes the capacitive output loading problem.

Cheers, James Arthur

Did you read the article? Eric Bogatin is a well-respected SI expert. He gives the explanation of ringing coax in the article. I copy it here:

------------------------------------------------------------------------- The root cause of the ringing is reflections from the mismatch at the RX between the impedance of the line and the high input impedance of the RX, and the mismatch when the reflected signal makes its way back to the low output impedance of the TX.

The reflection coefficient is close to 1 at the RX, but is a negative

reaches the RX, it reflects. The initial voltage at the RX is high. The reflected signal makes its way back to the TX, where it sees a high to low impedance and reflects. But the reflected voltage is negative.

This negative reflected signal makes its way back to the RX, where it again reflects, but since it is negative, pulls the signal at the RX down. When it reflects from the RX, it is still negative, makes its way back to the TX, changes sign when it reflects, and travels back to the RX as a positive signal.

From the initial positive signal at the RX, the signal takes one TD (time delay) to make it back to the TX, another TD to reflect back to the RX

the TX again, and another TD for the reflected signal to make it to the RX as a positive signal.

This is a rather surprising result: four time delays between successive peaks due to reflections in an unterminated line.

-------------------------------------------------------------------------

The difference between Eric's analysis and George's photos is I believe that George is measuring the ringing at the transmitting end, not at the unterminated receiving end.

However, the fact there is no ground plane and the photodode has such high capacitance could turn the multiple reflections into damped ringing, as shown in the pictures.

Adding a termination resistor to the coax stops the reflections. This could stop the ringing by eliminating the reflections and also by loading the op amp with a resistive load.

Len[inches]:

it first

D to get to

No I was looking at the unterminated receiving end... coax plugged into 'sc ope.

Ground plane, smound plane. It's only ~10 MHz, ground comes in on the powe r supply line, connects to the negative power bypass caps, non-inverting input, goes under opamp to the positve supply caps and coax ground. All told may 1" of trace. (It use to go to the photodiode(s) too, but now reverse biased.) I don't think a ground plane is going to change much. Unless it helps shield the stray FB capacitance.

George H.

If you want the lowest capacitance use several resistors in series. That or a really long high voltage version are about the only way to drop down the capacitance.

Ok but there are other options. For example, you can pick an amp with lower noise for the TIA, use a commensurately lower value for the feedback resistor and follow that TIA with a regular amp that makes up the difference.

Thanks Joerg, It looks like most of the capacitance is in the switch. Why doesn't Phil H. build PD amps with switched gain stages? I've got my answer. :^)

George H. (PCB's from advanced circuits are here, so on to some 'real' work.)

Yup. As JL put it, "there's no good place for the gain switch in a front end." I did a 1G/50G TIA for an output called Qcept (unfortunately now defunct). It used a non-latching relay to drive the coil of a latching one via capacitors. The coil of the latching relay was bootstrapped to get rid of the ~0.4 pF coil-to-contact capacitance.

Interestingly, the 1G resistor had to be kept shorted when not in use, because otherwise its noise current coupled through the capacitance of the open relay contacts and destroyed the measurement on the 50G range.

This TIA was for a capacitive surface-voltage probe for detecting sub-monolayer organic contamination on Si wafers.

With a PD, one thing you can do is to put TIAs on both ends and switch out the one you aren't using. A relay contact to AC ground doesn't usually make too much of a mess.

Cheers

Phil Hobbs

Hmm I think you mentioned the two TIA trick before.

I was thinking that I could make a series string of gain resistors rather than the parallel connection I have now. I drew that up and if I've got the stray C in the right place, then I think it will be a little better. (factor of two or so.) 'cause now at the low R end the stray C is shunting a smaller resistor, doing less harm.

George H.

This worked pretty well, switching the base voltages of the cascode transistors to select an amplifier.

The only thing that didn't work well was the customer. After I showed them how to do it, they did it themselves.

A relay would probably have worked there too, to switch the PD between TIA inputs.

Nice idea. BFT25As are at least as good as relays for that job, as long as you're using cascodes.

Cheers

Phil Hobbs

Multiple TIAs is the proper way to gain switch.

So what are those inductor values at the bases of the BFG25 switch transistors? 1.1 kilohenry? :-)

I had one new client who had just signed my consulting agreement. They wanted me to design a low frequency beamformer. I couldn't really see why but the customer is always king. We had a few minutes on the phone to discuss how the project could be started. I asked the group "Have you considered just using a plain vanilla multi-channel digitizer and doing all this in software?" ... loooong silence ... "Oh. OH! Let's think about that!". My total billable time for that client was 15 minutes which I didn't bother to bill.

Only Reeds. With others that might result in customer calls such as "It only works if I hit it with a ballpeen hammer a few times".

Reed relays are terrible. I like the Fujitsu FTR-B3GA4 series. They are good RF switches, too.

They do spec a min load though which usually means that long term issues can result if used for zero-current signal path switching:

I have bench gear with similar relays in there. Receivers used for EMC work and such. Everything was fine for about 15 years and then the first ones started not always making contact and I had to swicth back and forth. Maybe the gold layer had worn off.

With Reeds I so far didn't have much trouble. They are nice in a hostile environment because of encapsulation. I did wear one out but that was using it to drive a transmitter for fast morse code transmissions. I probably exceeded the maximum number of cycles big time.