CMOS analogues switch capacitance
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From: Joerg on 15 Oct 2006 17:23 Hello Chris, > > One other thing (that you would know well) is that the gate capacitance of > the MOS devices will depend strongly on the bias (VGS) so it is important > to measure the calibration channel and the measurement channel with the > same DC bias and about the same signal amplitude on the switch pins. > Another thing to watch is the substrate, for chips where it is brought out to a pin. During many of my consulting trips I found that much of the grief was rooted there. The substrate was connected to "somewhere" and in one case the pin was left open... -- Regards, Joerg http://www.analogconsultants.com
From: Jim Thompson on 15 Oct 2006 18:03 On Sun, 15 Oct 2006 23:02:08 +0100, Chris Jones <lugnut808(a)nospam.yahoo.com> wrote: >Jim Thompson wrote: > [snip] >> >> The only relatively constant term would be gate capacitance, ESD and >> MOS body diode capacitances will be all over the place with >> temperature. >> >> ...Jim Thompson > > >One other thing (that you would know well) is that the gate capacitance of >the MOS devices will depend strongly on the bias (VGS) so it is important >to measure the calibration channel and the measurement channel with the >same DC bias and about the same signal amplitude on the switch pins. > >Chris Nope. I've modeled that situation very carefully. Once a channel is induced, the gate _capacitance_ is essentially a constant, only the series resistance varies which, of course, can screw you up royally if you aren't aware of it. Now that I have it characterized I've been using gate capacitance as bypass capacitors extensively. ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Jim Thompson on 15 Oct 2006 18:05 On Sun, 15 Oct 2006 21:23:55 GMT, Joerg <notthisjoergsch(a)removethispacbell.net> wrote: >Hello Chris, > >> >> One other thing (that you would know well) is that the gate capacitance of >> the MOS devices will depend strongly on the bias (VGS) so it is important >> to measure the calibration channel and the measurement channel with the >> same DC bias and about the same signal amplitude on the switch pins. >> > >Another thing to watch is the substrate, for chips where it is brought >out to a pin. During many of my consulting trips I found that much of >the grief was rooted there. The substrate was connected to "somewhere" >and in one case the pin was left open... Ah, Yes! The floating body syndrome ;-) ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Joerg on 15 Oct 2006 18:19 Hello Jim, >> >>>One other thing (that you would know well) is that the gate capacitance of >>>the MOS devices will depend strongly on the bias (VGS) so it is important >>>to measure the calibration channel and the measurement channel with the >>>same DC bias and about the same signal amplitude on the switch pins. >> >>Another thing to watch is the substrate, for chips where it is brought >>out to a pin. During many of my consulting trips I found that much of >>the grief was rooted there. The substrate was connected to "somewhere" >>and in one case the pin was left open... > > Ah, Yes! The floating body syndrome ;-) > What floored me was the rationale behind it: "But we only need input, output and control signal". The topper of all unconnected things (grounds in that case) was when a sample-gate mixer was squealing and gurgling. I told them that shortwave stations were getting in because there isn't a ground structure. "Yeah, right. And they want to talk to us from outer space". Moved the L.O. registers a bit and BBC World Service came out of the speaker nice and clear... -- Regards, Joerg http://www.analogconsultants.com
From: Jim Thompson on 15 Oct 2006 18:27
On Sun, 15 Oct 2006 22:19:00 GMT, Joerg <notthisjoergsch(a)removethispacbell.net> wrote: >Hello Jim, > >>> >>>>One other thing (that you would know well) is that the gate capacitance of >>>>the MOS devices will depend strongly on the bias (VGS) so it is important >>>>to measure the calibration channel and the measurement channel with the >>>>same DC bias and about the same signal amplitude on the switch pins. >>> >>>Another thing to watch is the substrate, for chips where it is brought >>>out to a pin. During many of my consulting trips I found that much of >>>the grief was rooted there. The substrate was connected to "somewhere" >>>and in one case the pin was left open... >> >> Ah, Yes! The floating body syndrome ;-) >> > >What floored me was the rationale behind it: "But we only need input, >output and control signal". > >The topper of all unconnected things (grounds in that case) was when a >sample-gate mixer was squealing and gurgling. I told them that shortwave >stations were getting in because there isn't a ground structure. "Yeah, >right. And they want to talk to us from outer space". Moved the L.O. >registers a bit and BBC World Service came out of the speaker nice and >clear... I didn't come up with it myself, but one of the more clever things I've seen is for HV and power-off tolerant inputs and outputs... the body of the P-channel ESD device is disconnected using a switch ;-) ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food. |