Hello,
My application concerns exciting and reading temperatures from 12 different NTC's, converting readings to 16 bit data and sending it to 28335 DSP.
Thermistor have +-0.05ºC accuracy spec, global accuracy should be within +-0.1ºC. It is a tight spec and requires careful planning and design.
I'll split the application circuit in different threads, so I can detail what I need and organize it better.
Signal first comes from previous circuits (not shown): NTC Therm -> INA330 -> LPF @ 2Hz -> XTR111
There are independent circuitry and PCB's for each of the 12 NTC's , INA330, LPF and XTR111 setup.
INA330 is supplied by LP2989AIM-5.0 (+5V), which in turn is powered by a +12V DC external isolated supply and all circuitry of all PCB's share the same (isolated) ground. LPF is an OPA2188, tuned to 2Hz, 2 poles, powered directly by the +12V DC external isolated supply.
Each of these PCB's are separated by about 50 cm, and are disposed in a noisy environment, being physically close to ~4500 W SMPS.
Signal from each of the 12 the LPF's need to be transmitted to a MUX, next comes ADC.
The distance from each of the PCB's and the MUX is ~ 1 meter. Although it may be considered a short distance for current transmission, we have chosen 4-20 mA transmission, so there should be no surprises, particularly about EMI and noise.
For the 4-20 mA transmission, we need a 3 wire system, since MUX is single ended, loaded with a precision grounded 250 ohms low ppm resistor (for plain simple current to voltage conversion, back to +1 to +5V) and share the same ground as all circuits so far. We have decided for XTR111.
So attached there is our setup for XTR111. XTR Regulator is not used. +5VRef comes from REF5050. "V_LPF to XTR" is ths signal from the LPF, ranging from 0 to +5VDC.
I did not understand the "additional filter" suggested in XTR111 datasheet, pg 16, figure 41, shown below. It was poorly explained and circuit is somewhat confusing.
Is the 500 ohms resistor in the circuit supposed to replace the R6 and R7 (15 ohms each) found in the gate controlled current limiter, also suggested in datasheet, pg 14, figure 37a (below) ??
I suppose the only current path would be through the 500 ohms grounded resistor, supposedly before the load. If that is supposed to work like that (correct me if this is not the case), I can't afford it, since I am working with a 12V supply. This 500 ohm + the 250 ohm load would need 15V @ 20 mA, which I don't have.
Otherwise, is the 500 ohms the load itself??? It should be indicated in the circuit...
Why there is a left open connection at the right of the circuit, after the 10nF filter capacitor ? Is it supposed to connect anywhere??
Another question: I am connecting the output of the resistive divider (between +5VRef and "V_LPF to XTR") directly to VIN. By doing this, I can't use a resistor in series with VIN matching RSET and cancelling bias current. I thought of buffering resistor divider output (+1 to +5V) and connecting it to a 2k5 resistor before VIN. For that, I'd need a new op amp in the circuit. Is it really worth?? What is the actual gain by doing it??
Please check the rest of the circuit, I have never worked with XTR111 before and we are a bit short in time.
One more thing: I can't find any MUX in TI portfolio that works comfortably with 20 mA load, Vsupply >= 12V, single ended, 1:4 or 1:8 channels. I am missing something or TI really doesn't offer it?
I am considering ADG1608, it seems to do the job. It would be, however, the only part that is not TI... :(
EDIT: Also please confirm that EF and REGS pull up can be tied to +5V, instead of +12V. It is easier to interface directly with the MUX at +5V.