Contradiction in datasheet: capacitance vs bandwidth? - Page 1
Forum section: electronics / projects

author: rs20
author_rank: Super Contributor
timestamp: June 05, 2016, 11:56:37 am
content: Hi,I'm confused by theADG601datasheet. On page 3, it claims a 180MHz bandwidth, and it also claims a 145pF capacitance on Cd and Cs when the switch is on (I assume this means a capacitance to ground?). However, if I look at the bandwidth test schematic (Fig 21 on Page 10), it shows that the test is done using 50 ohm transmission lines. But, reactance of 145pF at 180 MHz is only 6.1 ohms. How on earth can be insertion loss be better than 3dB when the switch acts like a near-dead short to ground? What am I missing? The two specs seem totally contradictory to me.

author: GEuser
author_rank: Frequent Contributor
timestamp: June 05, 2016, 01:33:33 pm
content: CD, CS (On) 145 pF typf = 1 MHzThat in bold has anything to do with it?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2016, 01:40:38 pm
content: How can 2 meters of RG-59 have ~0.4dB of insertion loss at 180MHz, yet 160pF of equivalent capacitance?Well, in this example, the key word is "equivalent", specifically low frequency equivalent. They're not measured at the same points.The same might be happening for the switch, but it seems doubtful they'd be able to synthesize enough inductance in that chip to keep its impedance constant. If they did, there certainly ought to be a specification for it: ESL and Zo.Fig.21 doesn't suggest any room for cheating. 180MHz would be correct if the system impedance were around 5 ohms (similar to the switch resistance). But they specify 50 ohms. The gain equation given may have some wiggle (the customary definition is not "Vs without switch", but Vs linked to Vo with a mating cable), but not 10x worth.If you must use this part, I would suggest sampling it. Measure it yourself, see what the actual frequency response is.The isolation is terrible up there, anyway, so you wouldn't use this part for RF. Purpose-made RF switch parts have more "on" insertion loss (lower capacitance, higher resistance switches), and usually a tee structure (for 3x better isolation). You certainly wouldn't use one for power switching; there are RF relays for that (or, you can simply avoid switching RF power altogether).Tim

author: rs20
author_rank: Super Contributor
timestamp: June 05, 2016, 02:20:32 pm
content: Tim,Thanks for your response. What I actually want to do is have a switch or disable-able buffer than can drive DC -- 30 MHz (preferably up to 100 MHz) at +/- 5V at 50mA into an (inductive) load, and then switch into Hi-Z mode with less than 200ns (smaller the better) turn-off time. I haven't found op-amps with disable functionality that are quite up to the task/fast enough with the turn off -- how would you approach this problem?The underlying application is a coil containing a sample (doing nuclear quadrupole resonance), which I want to drive strongly and then very quickly switch into "listening" mode.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2016, 08:43:21 pm
content: How would I approach the problem?Excellent question!Let's start at the bottom. Roll itall the way backto the physics.1. What are the atoms doing? Electromagnetism. What's that? Fields with a ratio of E to H. Which has an impedance. (Even more particular: the atoms themselves might have their own ratio of E/H. But we can't make antennas that small, so, we'll deal with whatever space is around them.)2. It would be nice to couple to that, rather than to a coil (which will shunt most of your signal with its inductance). For a bulk substance, about the best we can do is, well... a coil, or a capacitor. Or both, I suppose. But maximizing mutual (coupled) fluxes, in any case.3. We can at least maximize the coil's impedance, by putting pole pieces around the substance. This does need to be done carefully, because we don't want to introduce additional resonances (electronic, magnetic or mechanical*), or thermal noise (losses). Powdered iron would probably be a good candidate. The average permeability won't be very much (maybe 2 or 3), since the gap will be large to contain the sample. But at least the core doesn't need to be special (as long as it's mu_r > 20 or so, it's fine; and such low permeability cores will also have low losses).*Electronic resonances, such as LC equivalent, or transmission line, effects. Shouldn't be much from the core, because it won't be very conductive, or will be such a bulk that any self-resonance will be at quite high frequencies. Magnetic resonances can come from physics: EPR (electron paramagnetic resonance), particularly under bias (same as NMR, but with less mass so a higher frequency for a given B field). There may also be quirky things like the resonance of an average magnetic particle or crystal or domain, or the transition frequency of the core (which can be avoided by using a low loss, low mu_r material). Mechanical resonances arise from magnetostriction pushing the material around, and you get acoustic resonances reflected as peaks and dips in the electrical frequency response. Using a low-magnetostrictive material is probably a good idea.Both of these approaches, combined, probably only account for a few dB of gain, but if the signal is poor (or you want the extra resolution), it's worth it.For a sample of, say, a 1 cm cube of polyethylene, you'd put capacitance plates on opposite faces, and a coil around it (with the solenoid's axis parallel to the plates). Plates connect to the coil terminals.You can imagine that, for such a situation, you still aren't getting much. The impedance of the coil will be ~ohms at most frequencies, and the impedance of the capacitor, ~Mohms. The ratio of both of these, to the load impedance (which is not quite the impedance of free space, but a bit lower, due to its index of refraction), goes to show you just how poor the electrical coupling will be.4. Anyway, to get the signal into a system, you need to match the impedance. The coil turns are variable, so you can really make it whatever you want. Minding that, more turns take physically longer wire, so you can't go crazy with it. Probably, 50-300 ohms would be fine. (By the way, in the same way you can add more turns to the coil, you can add more plates to the capacitor, interleaving them throughout the material. Of course, this gets rather hard to do on a sample you need to be able to insert and remove at will...)So now we come to what youthoughtyou were asking about,the transceiver design.You always need an impedance match. The signal you're interested in, is coming from some sort of impedance, transmitted through space, and received by a coil. So work with that. Your transmitter will be pushing into this impedance (albeit dominated by the coil's inductance), and your receiver will be sensing from it.Probably, getting a decade of bandwidth will be reasonable, easy even. Two decades will be hard. Three, probably impossible. You'll need to decide how much you want to target, and over what range.Design the coil inductance for LF cutoff at the lower frequency limit, and estimate parasitic capacitance of the coil (or its first series resonant mode) to fall above the upper frequency limit. These are the L/R and R*C time constants against the resistance of the transmitter (as a Thevenin equivalent) or receiver.For frequencies inbetween, the transceiver / system resistance dominates over coil inductance, and you get a flat passband. (Of course, not really all that flat, but varying by a couple dB. There may be a slight peak or dip, if the coil has a more complicated response at some frequencies, or from anything nearby causing funny effects. You'll have to calibrate that away somehow.)5. Design the transmitter and receiver to look like <system impedance> ohms. Not strictly necessary -- you're not matching to the best impedance (which would be the complex conjugate, which would require a negative inductance over this bandwidth*), but it's something, and it's not so bad as throwing your hands up in the air and letting the circuit figure itself out. The best justification is that you're looking for the signal that appears in parallel with the coil (it's a transformer, after all), so at least as long as the coil's impedance is high enough to ignore, you can get more signal and less coil.*Which you probably can't construct with low enough additive noise, or resistive component, to be useful here. Anyway, stabilizing an impedance inverter/gyrator circuit at RF is probably pants.6. Optimize things. Instead of a voltage source into a resistor into the coil, for the transmitter, just run the voltage source straight away. Not really a big deal in this case. You can also push to lower frequencies, within the current limits of the source anyway (but you may not get as much signal level or SNR from the receiver, in the same range). It may be practical to reduce the system impedance, even to ~ohms perhaps, which can give you wider bandwidth (not so much restricted by capacitance of the coil) and allows you to use a very low noise front end, like a grounded base amplifier with a large junction BJT.And for switching, you definitely want some kind of switch, you can't leave the transmitter on with no drive. It'll be making noise if nothing else (or if a CV source, then shunting the return signal too). But as we've seen, a switch (especially a low-ohm one) is a turd. It would probably be a good idea to go away from op-amps entirely. Why not use a single BJT? You probably don't need painfully high gain and accurate output. An emitter follower is a fine voltage source, and it can be biased off easily. Or if you're going even more crude than that (say, a MOSFET half bridge), it can simply be left 'off' (or tristated).The receiver switching is the next problem. You want strong attenuation during the transmitter pulse, so you don't overload any stages in the chain. You might use a more modest RF type switch, or even the ADG601 if the system impedance is low.For both transmitter and receiver, you will probably want to avoid any kind of unbalanced switch: anything that requires charging or discharging capacitors, or shifting unbalanced bias voltages or currents, will take forever to settle (on the order of the LF cutoff frequency or lower). This is a nice advantage of an open-collector or open-drain transmitter (when off, it's simply open circuit), and a receiver could be designed the same way.For example, if you go with a grounded-base stage, it can be turned off by reverse-biasing the base; but that would take time to charge the required bypass cap. You could put two together, out of phase, so you get a diff pair. Except, instead of input across the bases, the bases are tied and input is across the emitters, via centertapped transformer winding. CT goes to GND via bias resistor. Collectors go to a transformer for output coupling. By varying base voltage (which has, ideally, no common mode signal on it, so can be switched on and off quickly), you vary bias current; it's a single balanced mixer (or a two quadrant multiplier, or an OTA without the DC coupling).For that matter, you can build the same thing with two transformers and two diodes (a SBM with the bias switched from positive "short it together" current, to reverse bias); you don't incur mixer loss (6dB) because the signal's not being toggled on and off while you're listening. (Or even better: the loss is the part you WANT cut off. Correlation FTW!) You may not get very good cutoff due to diode capacitance though; nice part about a common-base SBM is the low Cce.Tim

author: rs20
author_rank: Super Contributor
timestamp: June 12, 2016, 12:59:19 pm
content: Hi Tim,Thanks as always for your amazingly detailed, useful and interesting replies.The traditional approach, as used by my father back in the day while he was doing his PhD, was to use a regenerative circuit type approach: the coil containing the sample was used as the L in the LC tank of an oscillator (with a variable capacitor). The oscillator was allowed to run, and then the tank was briefly quenched/shorted out to kill the oscillations. Any echos coming back from the sample would help re-bootstrap the oscillator more quickly, so whenever the frequency set by the LC tank happened to land on a nuclear quadrupolar resonance frequency, the oscillations would reappear sooner than otherwise. Back then, the graphs produced by the pen plotter had to be manually stopped, the oscillator frequency read off using a frequency meter of some sort, and then the rest of the frequencies on the graph were worked out using manual interpolation. My father commented how much he would have liked to have a computer that could generate given frequencies programmatically, and my pointing out that such chips are now $10 in a 5x5mm package is pretty much the spark for doing this project! As well as gaining experience with all this DDS and PLL stuff. FWIW, the sample I'll be using is copper(I) oxide, which has resonances at the 24 and 26 MHz marks (the two isotopes of copper!)I'm interested by the idea of using capacitor plates as the pick-up mechanism as opposed to a coil. Not sure at all how common that approach is, I was only aware of using a second coil.Thanks for the idea of using pole pieces, at the very least I'll make sure my box slides together in such a way that that is easy to add in later.I've had a go at using an emitter follower approach for the Hi-Z functionality. The right-hand half of this diagram is of more interest than the left, I think. I decided to just clamp the receiver input rather than explicitly switch it in. Yhe output is ultimately (after a couple more stages of amplification) going into a Picoscope. In the interests of keeping noise low, and having an approximately OK impedance match, I used a fairly small 330R for the receiver amp and a rather schmicko low-noise op-amp ( < 1nV/rtHz, although I fully appreciate that the noise from the 330R resistor will swamp the op amp!) The rest of it is just converting from differential current to single-ended voltage. I'm totally OK with the design being expensive and "over-engineered", but I'd be very interested to hear what your greatest concerns are with this design. My guess is that you'd like the 330R (R14) resistor to be smaller :-)Note that the 1p and 0.1p capacitors are placeholders, I'll be setting those through experimentation on the PCB.DatasheetsAD9102-- AWG/DACEL5105-- Drive opamps2SC5707-- Drive NPNsLMH6629-- Receiver ultra-low-noise opampPS/ How much noise does a switched-off NPN transistor generate? I'm hoping it's like a very high valued resistor, i.e. very little noise current.EDIT: Fixed broken image link.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 12, 2016, 04:46:35 pm
content: Capacitive plates of course won't do much on conductive material. I suppose it depends on how doped your copper oxide sample is.As shown, you'll have the problem where the ~50pF Ccb couples the "turn off" edge into the tank.That also means the tank doesn't see a resistive 330 ohm) load, but is left with ~100pF in parallel. 100pF resonates with 3u at 9.1MHz (giving a lowpass cutoff), with Zo = 173 ohms, so it will be somewhat damped by the 330 ohms.3u and 47n resonates at 420kHz, but is that a lower cutoff (you just don't want anything on the coil below ~400kHz), or an intended gain factor, within the bandwidth? Zo is 8 ohms, so it won't be at all well-damped during drive (which is very low impedance, r_e of the transistor!).Tim

author: rs20
author_rank: Super Contributor
timestamp: June 15, 2016, 01:08:28 pm
content: Thanks for pointing out the capacitance issue. I've substitutedBFU590Gs in (with aBAW56LT3Gto protect the delicate base), to replace the overly capacitative BJTs I had before.47n is just there to act as a DC blocker (any small offset in the driver leads to a forever-increasing current ramp otherwise), and I had just made it "rather large" for that reason. You raise a good point, I'll have to think how to dampen any oscillations there. Not sure why I'd particularly care during driving, but it'd be an unpleasant issue if it continued to ring while receiving.Thanks again! I'm well into PCB design now.