Resistor ratios

You don't say if you're using the resistors to program a current source or what, but if you're willing to make an R-2R ladder you can get

For that matter, you can make a DAC -- but you'd need good resistors.

Well, 249/499/1000/2000 is close, and

Cheers

Phil Hobbs

If it comes to it, just pick a stock value, and do two-in-parallel, one, two-in-series.

Tim, Phil, thanks. I had a total brain fart today. It's an opamp -> fet -> load/resistor_feedback current source. I wanted to switch R (with rotary switch) and I thought I could do it in shunt/ parallel, until I tried to lay it out.. oops. (I hope the odor did not escape my room.)

Max current is in the 200mA range. (limited by switch...) Now I'm thinking about ditching the rotary switch and using a latching relays.. ~$2-3 each (in 100) Seems like I could do a lot with 3-4. I'd still have to have inputs.. push buttons I guess.. and led indicators. A front panel pcb... (rotary switches are on the shelf.)

Sorry for thinking out loud.

George H .

PS, Phil, Re: 249,499, 1k, 2k, 4.02k I wish we could "bend the log a bit" and have 250,500,1k,2k,4k..

GH

Can you do the current control on the input side, rather than switching 200 mA? Use an analog mux or a low-current rotary switch.

Does anybody still make R-2R ladder networks? I guess you could make your own.

Use a DAC?

Yeah it would be much better to do most of the switching at the front end.... I need less than 2 decades. (And I'm feeling like someone will want more current.)

Thanks.. I was stuck looking at wrong thing.

George H.

You can bend them a bit by using normal resistors as a small perturbation, e.g. 249 or 499 ohms 0.1% + 1 ohm 1%, 4.02k 0.1% // 806k 1%.

Cheers

Phil Hobbs

DigiKey has a bunch of DAC0808s in stock. It looks like it's not the least bit dead.

There are tons of DACs around, so George could do something to map his rotary switch into DAC codes. Actually, it's simple 1:1 if he wants

A chopamp would have essentially zero offset error.

My recollection is that R-2R ladder DACs are far less sensitive to resistor tolerance than 1-2-4-8R-type DACs. Back in the Olden Days of DOS computer audio, an 8-bit R-2R ladder made from 10K-20K 5% resistors (driven by a printer port... remember those?) gave pretty decent sound.

You could even make an 8-bit ADC by wiring the ladder output to a status input pin on the port. Wire the input signal through a summing resistor to the same pin, and use SAR logic to balance the ladder output against the signal at the gate threshold voltage. The whole thing could be wired into the parallel port connector hood to get a "passive" ADC (since the active part was inside the computer).

Ahh, DOS were da days!

Best regards,

Bob Masta DAQARTA v9.00 Data AcQuisition And Real-Time Analysis

I've used an AD7541 in the distant past... "Gack", I didn't know they were so expensive!

George H.

I was playing with this idea in bed last night... The onerous part (for me) would be the user interface. Push buttons and led indicators.. It's not something I've done, so I'd have to research, copy and adapt, screw-up, etc...

The switch is too easy... I do still have a bug up my butt with grayhill switches.. so doing something else is appealing.

George H.

You and I are thinking alike here.

Yes, but if you want 2:1 ratios, then all you need to do is turn on one pin of the DAC at a time, in order.

RE: Chop amp.

Say I was looking through AoE3 for a precision opamp with a bit of speed, and I cam across the opa192.

Which has got some sweet specs! If someone mentioned this opamp before then I missed it. Except for this weird extra noise when the input is near the positive rail, it looks almost "perfect".

George H.

There is something steampunk and nostalgic about rotary switches. But the switch and knob can get expensive these days, not to mention wiring.

Yikes, that is pricey. Serial DACs are cheap, but I guess you want parallel. We're paying $4.40 for AD5440, a dual 10-bit parallel DAC. There are probably cheaper singles, maybe 8 bits. That DAC0808 is 69 cents in quantity and might be accurate enough.

Chopamps: ADA4638 handles +-15 supplies and costs about $2. Lower voltage chopamps like AD8628 are cheaper.

I know. It's lowering, but every time I end up doing a full-system design the case, knobs and buttons end up costing more than the circuitry.

Well, buy more expensive parts!

We've found that designing our own box, and having a shop punch/bend/anodize the sheet metal, is cheaper than buying a commercial box, much less machining it.