On 2020-08-18 20:15, Chris Jones wrote:> On 19/08/2020 01:27, Phil Hobbs wrote: >> I'd really like to use a dpot though, because that can be programmed >> at test time whereas using a mux would mean I'd need a micro on a >> hitherto all-analogue board. > > If you're going to program it at test, could you put two or three DPOTs > in series (so less voltage across each one), and power them either with > photovoltaic isolators, or some fancy op-amp bootstrapping circuit. Yes > the SPI level shifters could get interesting, but those could be a > one-off monstrosity residing in your tester, behind the pogo pins or > whatever.Not a bad idea--I'm not at all above such things. In a tester, it could be three Bus Pirates hung off USB isolators, so nobody would even have to know. ;) This is one of those things that comes up a fair amount though, so it would be useful to have a solution that would work in a box with a uC as well. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Transformer-coupled dpot
Started by ●August 17, 2020
Reply by ●August 18, 20202020-08-18
Reply by ●August 19, 20202020-08-19
On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>So I'm doing a spin of my swoopy nanoamp photoreceiver board, to make it >smaller and more versatile. > >The original QL01 has two pots, one for offset and one for HF boost. >The HF boost is a lead-lag network that attenuates the voltage the FB >resistor sees, reducing the effect of its parallel capacitance. > >The first one can easily be a dpot, but the second one is a bit more of >a challenge. It's not too hard to level-shift unidirectional I2c up to >the op amp's output--a PNP CB stage with the base grounded, followed by >an NPN CB stage with the base about 3V above the negative supply rail, >as in the LTspice files below (i2cls.lib and I2Cshifter.asc. The dpot >can be an AD5273BRJZ1, whose 6 MHz bandwidth will work fine. > >Problem is, at high frequency most of the signal swing appears across >the dpot, so if I do it this way I'll have to reduce the supplies. > >I'm considering two things: first, leaving the resistors fixed and >switching the caps in and out with a high-voltage mux, or else (more >interestingly) transformer-coupling the dpot. The first > >It'll need a fair amount of inductance to look like 1k ohms at 200 kHz >and above, probably a couple of mH, and a turns ratio of 2:1 or so to >reduce the voltage swing. > >Anybody here done something like that? > >Cheers > >Phil HobbsOh, there's another trick: a tiny surface-mount dip switch and 4 caps makes a 4-bit capacitive DAC. Or a tiny screwdriver-adjusted variable capacitor. Both are cheap and low noise and require no software. https://www.digikey.com/products/en?keywords=490-2008-1-ND We pay 26 cents for that one.
Reply by ●August 19, 20202020-08-19
On 2020-08-19 14:24, John Larkin wrote:> On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> So I'm doing a spin of my swoopy nanoamp photoreceiver board, to make it >> smaller and more versatile. >> >> The original QL01 has two pots, one for offset and one for HF boost. >> The HF boost is a lead-lag network that attenuates the voltage the FB >> resistor sees, reducing the effect of its parallel capacitance. >> >> The first one can easily be a dpot, but the second one is a bit more of >> a challenge. It's not too hard to level-shift unidirectional I2c up to >> the op amp's output--a PNP CB stage with the base grounded, followed by >> an NPN CB stage with the base about 3V above the negative supply rail, >> as in the LTspice files below (i2cls.lib and I2Cshifter.asc. The dpot >> can be an AD5273BRJZ1, whose 6 MHz bandwidth will work fine. >> >> Problem is, at high frequency most of the signal swing appears across >> the dpot, so if I do it this way I'll have to reduce the supplies. >> >> I'm considering two things: first, leaving the resistors fixed and >> switching the caps in and out with a high-voltage mux, or else (more >> interestingly) transformer-coupling the dpot. The first >> >> It'll need a fair amount of inductance to look like 1k ohms at 200 kHz >> and above, probably a couple of mH, and a turns ratio of 2:1 or so to >> reduce the voltage swing. >> >> Anybody here done something like that? >> >> Cheers >> >> Phil Hobbs > > Oh, there's another trick: a tiny surface-mount dip switch and 4 caps > makes a 4-bit capacitive DAC.I'm not at all above that sort of thing either. I could even use a hex dip switch to mimic a variable cap. ;)> Or a tiny screwdriver-adjusted variable capacitor. > > Both are cheap and low noise and require no software.Agreed. Part of what we're doing is to make our gizmos attractive for people to license, and as you know there's a lot of prejudice out there against manual trims. That's not completely unreasonable when the person doing the adjusting has no idea how the circuit works--it's easy to encapsulate the know-how in the test software. For protos and proof-of-concept systems I often use gimmick caps to adjust amplifier peaking.> https://www.digikey.com/products/en?keywords=490-2008-1-ND > > We pay 26 cents for that one.Long obsolete, unfortunately. Our existing trimpot works okay but doesn't play that well with ATE. (You used to be able to get GPIB-controlled screwdrivers for that sort of thing.) Plus, needing to hang an AC tweak on some out-of-the-way circuit node is something that comes up a fair amount round here. The transformers will be here today, so maybe next week I'll give it a whirl. We're ordering boards probably tomorrow, so it'll have to be trimpots for this batch. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●August 19, 20202020-08-19
On Wed, 19 Aug 2020 15:51:50 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 2020-08-19 14:24, John Larkin wrote: >> On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> So I'm doing a spin of my swoopy nanoamp photoreceiver board, to make it >>> smaller and more versatile. >>> >>> The original QL01 has two pots, one for offset and one for HF boost. >>> The HF boost is a lead-lag network that attenuates the voltage the FB >>> resistor sees, reducing the effect of its parallel capacitance. >>> >>> The first one can easily be a dpot, but the second one is a bit more of >>> a challenge. It's not too hard to level-shift unidirectional I2c up to >>> the op amp's output--a PNP CB stage with the base grounded, followed by >>> an NPN CB stage with the base about 3V above the negative supply rail, >>> as in the LTspice files below (i2cls.lib and I2Cshifter.asc. The dpot >>> can be an AD5273BRJZ1, whose 6 MHz bandwidth will work fine. >>> >>> Problem is, at high frequency most of the signal swing appears across >>> the dpot, so if I do it this way I'll have to reduce the supplies. >>> >>> I'm considering two things: first, leaving the resistors fixed and >>> switching the caps in and out with a high-voltage mux, or else (more >>> interestingly) transformer-coupling the dpot. The first >>> >>> It'll need a fair amount of inductance to look like 1k ohms at 200 kHz >>> and above, probably a couple of mH, and a turns ratio of 2:1 or so to >>> reduce the voltage swing. >>> >>> Anybody here done something like that? >>> >>> Cheers >>> >>> Phil Hobbs >> >> Oh, there's another trick: a tiny surface-mount dip switch and 4 caps >> makes a 4-bit capacitive DAC. > >I'm not at all above that sort of thing either. I could even use a hex >dip switch to mimic a variable cap. ;) > >> Or a tiny screwdriver-adjusted variable capacitor. >> >> Both are cheap and low noise and require no software. > >Agreed. Part of what we're doing is to make our gizmos attractive for >people to license, and as you know there's a lot of prejudice out there >against manual trims. That's not completely unreasonable when the >person doing the adjusting has no idea how the circuit works--it's easy >to encapsulate the know-how in the test software. > >For protos and proof-of-concept systems I often use gimmick caps to >adjust amplifier peaking. > >> https://www.digikey.com/products/en?keywords=490-2008-1-ND >> >> We pay 26 cents for that one. > >Long obsolete, unfortunately.Maybe that's why we have a ton of them. Probably an EOL buy.> >Our existing trimpot works okay but doesn't play that well with ATE. >(You used to be able to get GPIB-controlled screwdrivers for that sort >of thing.) > >Plus, needing to hang an AC tweak on some out-of-the-way circuit node is >something that comes up a fair amount round here. > >The transformers will be here today, so maybe next week I'll give it a >whirl. We're ordering boards probably tomorrow, so it'll have to be >trimpots for this batch. > >Cheers > >Phil HobbsI still think you could use the dpot as a grounded pot, but I don't entirely understand your situation.
