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my

you want 4 wire for 100R RTD, they are only ~0.4V/C so wiring matters

I'd just do ~1K resistor top and bottom and feed it from a 5V reference, but it requires that you have enough bits to linearize in software

I've mostly used 1K RTDs where you can get aways with two wires, but it is real important that the impedance on the two wires are similar or the common mode rejection sucks

-Lasse

I meant the current flow through the diode if the LDO accepts a sink current on his output, most of them did not AFAIK.

Habib

It warns about going 50 millivolts past the rails!

These parts are similar. They are both an astounding bunch of technology for about $6.

Oh, the +5 volts is the gazillion-amp VME bus power supply. No problem there.

The 3.3 volt clamp below could have a problem if the customer applies a bunch of voltage to all 16 channels. I'll need to take care of that.

Oh i see ! i forgot you are VME's specialists.

Last time i designed a 3U slave VME card ... it was in 1991 or something like that, also i remember the old MVE motorola CPU module, it was fun programming on OS/9 !

BR, Habib.

alog

And

RTD_Input.JPG

..

RTD_Input_2.JPG

how about something like this? (could make some more fancy v-regs if needed )

Version 4 SHEET 1 1036 1220 WIRE 96 -368 -128 -368 WIRE 176 -368 96 -368 WIRE 720 -368 176 -368 WIRE 848 -368 720 -368 WIRE 848 -352 848 -368 WIRE -128 -288 -128 -368 WIRE 96 -256 96 -368 WIRE 720 -224 720 -368 WIRE 848 -208 848 -224 WIRE 176 -160 176 -368 WIRE -128 -128 -128 -208 WIRE 96 -112 96 -176 WIRE 112 -112 96 -112 WIRE 848 -112 848 -144 WIRE 96 -48 96 -112 WIRE 176 -32 176 -64 WIRE 240 -32 176 -32 WIRE 720 -16 720 -144 WIRE 720 -16 560 -16 WIRE 96 32 96 16 WIRE 176 48 176 -32 WIRE 720 80 720 -16 WIRE 848 128 848 -48 WIRE 848 128 784 128 WIRE 848 144 848 128 WIRE 96 240 96 224 WIRE 176 240 176 128 WIRE 176 240 96 240 WIRE 720 240 720 176 WIRE 848 240 848 224 WIRE 848 240 720 240 WIRE 848 256 848 240 WIRE 240 416 240 -32 WIRE 336 416 240 416 WIRE 560 416 560 -16 WIRE 640 416 560 416 WIRE 240 464 240 416 WIRE 560 480 560 416 WIRE 0 544 -128 544 WIRE 240 544 240 528 WIRE 240 544 80 544 WIRE 560 544 240 544 WIRE 960 544 560 544 WIRE -128 640 -128 624 WIRE 336 672 336 416 WIRE 336 672 240 672 WIRE 240 736 240 672 WIRE 640 752 640 416 WIRE 640 752 560 752 WIRE 0 816 -128 816 WIRE 240 816 240 800 WIRE 240 816 80 816 WIRE 560 816 240 816 WIRE 960 816 560 816 WIRE -128 912 -128 896 WIRE 336 944 336 672 WIRE 336 944 240 944 WIRE 240 976 240 944 WIRE 640 992 640 752 WIRE 640 992 560 992 WIRE 0 1056 -128 1056 WIRE 240 1056 240 1040 WIRE 240 1056 80 1056 WIRE 560 1056 240 1056 WIRE 960 1056 560 1056 WIRE -128 1152 -128 1136 FLAG 96 240 0 FLAG -128 -128 0 FLAG 848 256 0 FLAG -128 640 0 FLAG -128 912 0 FLAG -128 1152 0 SYMBOL npn 112 -160 R0 SYMATTR InstName Q1 SYMATTR Value 2N2222 SYMBOL schottky 80 96 R0 SYMATTR InstName D1 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 80 -272 R0 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL voltage -128 -304 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL pnp 784 176 R180 SYMATTR InstName Q3 SYMATTR Value 2N2907 SYMBOL schottky 832 -288 R0 SYMATTR InstName D2 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 832 128 R0 SYMATTR InstName R2 SYMATTR Value 1k SYMBOL schottky 80 32 R0 SYMATTR InstName D3 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 832 -208 R0 SYMATTR InstName D4 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 224 464 R0 SYMATTR InstName D5 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 576 544 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D6 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 96 528 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 1k SYMBOL voltage -128 528 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value SINE(0 10 1) SYMBOL schottky 80 160 R0 SYMATTR InstName D7 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 832 -112 R0 SYMATTR InstName D8 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 224 736 R0 SYMATTR InstName D9 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 576 816 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D10 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 96 800 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 1k SYMBOL voltage -128 800 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V3 SYMATTR Value SINE(0 7 2 0 0 90) SYMBOL schottky 224 976 R0 SYMATTR InstName D11 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 576 1056 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D12 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 96 1040 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 1k SYMBOL voltage -128 1040 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V4 SYMATTR Value SINE(2.5 2.2 2 0 0 90) SYMBOL schottky 80 -48 R0 SYMATTR InstName D13 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 832 -352 R0 SYMATTR InstName D14 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 704 -240 R0 SYMATTR InstName R6 SYMATTR Value 1k SYMBOL res 160 32 R0 SYMATTR InstName R7 SYMATTR Value 1k TEXT -162 664 Left 2 !.tran 1

Three days checking layouts? Ug! That's a *lot* of time.

This is barbaric, but it's a start.

A shunt reg would work, but I'd have to dump in more current than the stuff on the board would need.

Or, I could make a dedicated clamp bus, which is easy but needs to be routed to all the channels. Similar to your circuit, PNP plus bandgap, maybe even closed loop. That isn't bad.

An LM337 running upside-down could be a voltage-regulated current sink. That's easy.

come in 24 bits. In real life, they can deliver 10 PPM accuracy, so the RT D

he

Sigma-delta ADC's use serial output, and thus tend to deliver 3 three 8-bit bytes of data - 24 bits.

In reality they don't seem to do better than 20-bits (1 ppm). We had a disc ussion about this here a few years ago.

y will be a lot better than the RTD accuracy, so it all works.

That does over-simplify the situation a fair bit. One of the gotcha's with using RTD's at high precision is that you have to AC excite them (or at lea st use reversing DC). The thermocouple voltages along the leads otherwise s tart getting embarrassing when you get down to millidegree levels.

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ate,

es/RTD_Input.JPG

ke...

es/RTD_Input_2.JPG

y.

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could work but then you'd have to worry if it behaves when used like that

a bandgap is probably overkill, the supply for the adc is your reference so a divider or a diode could do

Version 4 SHEET 1 1272 1220 WIRE -80 -368 -272 -368 WIRE 0 -368 -80 -368 WIRE 176 -368 0 -368 WIRE 720 -368 176 -368 WIRE 1072 -368 720 -368 WIRE 1168 -368 1072 -368 WIRE -272 -288 -272 -368 WIRE -80 -288 -80 -368 WIRE 720 -224 720 -368 WIRE 176 -160 176 -368 WIRE 0 -144 0 -368 WIRE -272 -128 -272 -208 WIRE -32 -128 -48 -128 WIRE 112 -112 32 -112 WIRE -80 -96 -80 -208 WIRE -32 -96 -80 -96 WIRE 176 -32 176 -64 WIRE 240 -32 176 -32 WIRE 720 -16 720 -144 WIRE 720 -16 560 -16 WIRE 1136 -16 720 -16 WIRE -48 0 -48 -128 WIRE 176 0 176 -32 WIRE 176 0 -48 0 WIRE 176 48 176 0 WIRE 720 80 720 -16 WIRE 1072 96 1072 -368 WIRE 1136 112 1136 -16 WIRE 1136 112 1104 112 WIRE 1040 128 784 128 WIRE 1168 144 1168 -304 WIRE 1168 144 1104 144 WIRE -80 176 -80 -96 WIRE 1168 176 1168 144 WIRE 176 240 176 128 WIRE 720 240 720 176 WIRE 240 416 240 -32 WIRE 336 416 240 416 WIRE 560 416 560 -16 WIRE 640 416 560 416 WIRE 240 464 240 416 WIRE 560 480 560 416 WIRE 0 544 -128 544 WIRE 240 544 240 528 WIRE 240 544 80 544 WIRE 560 544 240 544 WIRE 960 544 560 544 WIRE -128 640 -128 624 WIRE 336 672 336 416 WIRE 336 672 240 672 WIRE 240 736 240 672 WIRE 640 752 640 416 WIRE 640 752 560 752 WIRE 0 816 -128 816 WIRE 240 816 240 800 WIRE 240 816 80 816 WIRE 560 816 240 816 WIRE 960 816 560 816 WIRE -128 912 -128 896 WIRE 336 944 336 672 WIRE 336 944 240 944 WIRE 240 976 240 944 WIRE 640 992 640 752 WIRE 640 992 560 992 WIRE 0 1056 -128 1056 WIRE 240 1056 240 1040 WIRE 240 1056 80 1056 WIRE 560 1056 240 1056 WIRE 960 1056 560 1056 WIRE -128 1152 -128 1136 FLAG 176 240 0 FLAG -272 -128 0 FLAG 720 240 0 FLAG -128 640 0 FLAG -128 912 0 FLAG -128 1152 0 FLAG 1072 160 0 FLAG 0 -80 0 FLAG 1168 256 0 FLAG -80 240 0 SYMBOL npn 112 -160 R0 SYMATTR InstName Q1 SYMATTR Value 2N2222 SYMBOL res -96 -304 R0 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL voltage -272 -304 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value 5 SYMBOL pnp 784 176 R180 SYMATTR InstName Q3 SYMATTR Value 2N2907 SYMBOL res 1152 160 R0 SYMATTR InstName R2 SYMATTR Value 1k SYMBOL schottky 224 464 R0 SYMATTR InstName D5 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 576 544 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D6 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 96 528 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 1k SYMBOL voltage -128 528 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value SINE(0 10 1) SYMBOL schottky 224 736 R0 SYMATTR InstName D9 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 576 816 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D10 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 96 800 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 1k SYMBOL voltage -128 800 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V3 SYMATTR Value SINE(0 7 2 0 0 90) SYMBOL schottky 224 976 R0 SYMATTR InstName D11 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 576 1056 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D12 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 96 1040 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 1000 SYMBOL voltage -128 1040 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V4 SYMATTR Value SINE(2.5 2.45 2 0 0 90) SYMBOL res 704 -240 R0 SYMATTR InstName R6 SYMATTR Value 100 SYMBOL res 160 32 R0 SYMATTR InstName R7 SYMATTR Value 100 SYMBOL Opamps\\LT1466 0 -176 R0 SYMATTR InstName U1 SYMBOL Opamps\\LT1466 1072 64 M0 SYMATTR InstName U2 SYMBOL schottky -96 176 R0 SYMATTR InstName D1 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 1152 -368 R0 SYMATTR InstName D2 SYMATTR Value BAT54 SYMATTR Description Diode SYMATTR Type diode TEXT -162 664 Left 2 !.tran 1

-Lasse

OK, this would work, two parts that we have in stock.

That's a good one. Thanks