PHEMTs have insanely low Cd-g. A phemt would be best used as a shunt switch to ground, where it would only need about a volt of gate swing, and would short a signal to ground, but it couldn't handle much signal swing.
So, I'm thinking about a 2-opamp closed-loop thing, with an integrator, that has a phemt in there to short out the signal during the glitch, essentially killing loop gain during the glitch. It would, overall, have a 1st order lowpass response, or it could be configured as a slew rate limiter.
Something like that.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
My very first "real" project in this direction was my masters project. I've got to scan that stuff in some day. Anyhow, it was the design of a CCD camera from scratch because the circuit from the CCD manufacturer was ... ahem ... lets say, the pits. They got a measly 45dB of dynamic range because of bad charge injection noise and such.
So I used diode quads, one for each of the three output channels. Off the bat that bumped it to 60dB. Charge injection wasn't even measurable anymore and we were down to the inherent noise of the CCD "bucket-brigade" cells.
The trick: The transformers driving the sampler were carefully made not to favor one side over the other, so any capacitive coupling would be neutralized. I made them out of ferrite, cost almost nothing. We did have a DC offset due to diode imbalances but that was a piece of cake to compensate for (I just clamped it away).
If a FET switch is desired one could either look for well neutralized mux ICs or counter-inject a charge of opposite polarity. The usual, two bent wires near each other as a poor man's capacitor, bent with a wooden chop stick until it's just right. A good excuse for an evening at the Japanese restaurant.
Instruments for the past 15 years or so. Less is more, no T/H is used. The idea there is to uniformize the glitch energy, or make it code independent, and shift its spectrum to the sampling frequency where the so-called anti-imaging LPF can eliminate it. This can't be done with code-dependent glitch energy since quite a lot of it remains withing the signal band. See US5646620...guess it's still protected though.
What I had in mind was making the slew _time_, to negotiate one step increment, slightly longer than the glitch width.
I've done it in CMOS. With the advantage that I can make a current output stage (with bounds) OpAmp, compensated by the slew cap :-) ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Instruments for the past 15 years or so. Less is more, no T/H is used. The idea there is to uniformize the glitch energy, or make it code independent, and shift its spectrum to the sampling frequency where the so-called anti-imaging LPF can eliminate it. This can't be done with code-dependent glitch energy since quite a lot of it remains withing the signal band. See US5646620...guess it's still protected though.
If you can balance the drive currents really, really well.
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
Instruments for the past 15 years or so. Less is more, no T/H is used. The idea there is to uniformize the glitch energy, or make it code independent, and shift its spectrum to the sampling frequency where the so-called anti-imaging LPF can eliminate it. This can't be done with code-dependent glitch energy since quite a lot of it remains withing the signal band. See US5646620...guess it's still protected though.
When would you reset it?
--
John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro acquisition and simulation
"Balance" is not critical if you use an OpAmp. ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Instruments for the past 15 years or so. Less is more, no T/H is used. The idea there is to uniformize the glitch energy, or make it code independent, and shift its spectrum to the sampling frequency where the so-called anti-imaging LPF can eliminate it. This can't be done with code-dependent glitch energy since quite a lot of it remains withing the signal band. See US5646620...guess it's still protected though.
I've used low bit count DACs and past them through an integrate that is controlled with a JFET on another IO port. When output value is shifted beyond the jitter point, the integrator id switched off.
Jfet is in the integrator feed back loop.
This all assumes we have a uC of some sort behind the scenes.
Yep, per the designer it's the internal switches. Glitch is the price paid for a high-voltage part, as the switches are necessarily bigger (= higher capacitance) in a higher-voltage process.
Is the problem the area of the glitch or the height? If the height, then you may be able to simply filter the output with a cap since output resistance is ~3k. If it's area, you're stuck, a filter cap won't change that. In that case you'd need a deglitcher, no mean feat at 20bits. At that level you're looking at expensive caps like porcelain, and even then you might need trims for dielectric absorbtion. FR-4 could be problematic as well. I did a true
16-bit chip-and-wire hybrid SHA many moons ago (like 300 = 25 years) and it had two DA trims even with a porcelain hold cap on a ceramic substrate.
He also said to make sure you have the latest silicon version. A fix following introduction that reduced glitch energy. I don't know if there's a difference in brand, or if it's just date code, sorry. If I weren't in the midst of moving office I could probably figure out who to ask, but not this week.
Cascoding the charger transfer device was done back in the NMOS analog days. [Intel 6um double poly eprom process.] Once you went to CMOS, charge injection was less of a problem since you had some cancellation due to using both sexes of switches.
The old video DACs used a combination of binary weighting and thermometer coding. You still got glitches when the binary segments were switched.
Run two "in parallel", take only the glitches from one, invert and sum that wit output from the other. Will not be perfect,but that may do enough that a nice filter can fix.
On a sunny day (Wed, 18 Jul 2012 17:18:23 -0400) it happened Spehro Pefhany wrote in :
Maybe the opposite can be done, open a series switch on the spikes. If you have the switch signal and it is early, use it to open a switch in the output for a few ns (longer than the transients). For the rest eitehr way it is homework, drive impedance, switch resistance, RC time, maybe need buffers, noise level, opamp choices.
I've simulated a passive notch filter that might be okay enough, especially if there is a significant reduction in glitching at the silicon level (have to simulate the signal a bit more accurately to be sure). I'm not sure DA is that important in this application- it will just add a bit of a tail to the response. Leakage, OTOH matters a lot.
Would love to know how to find out how to tell the chips apart when/if you get a chance.
Ordinary two op amp closed loop T/H, with a resistor and a couple of Schottky diodes to prevent the input amp from railing, dual-gate MOSFET connected S+G2 to the first op amp output, D to the hold cap, G1 is the track/hold control. Drive G1 negative during the glitch using an open collector with a resistor to S so that it returns to S potential while on.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
I had in mind this: an input error amp with modest gain, like 1, followed by an integrator, closed loop. The midway connection is schottky clamped, maybe not necesary for small glitches. The difference is that I'd propose to blank the glitch by shorting the midway signal to ground with a PHEMT, which probably has the best Ron*Cdg product that you can get anywhere. Cut the loop gain by 10:1 or so around glitch time.
The dual-gate will shoot some of the G1 charge back into the first opamp, which can cause problems there.
Making the second stage an integrator keeps the switch, whichever kind, always working near ground. That keeps things more linear, and makes charge cancellation, if it's needed, a lot easier.
Either way, the s/h is usually in track mode, briefly in hold mode at glitch time. So cap DA doesn't matter much.
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators
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