Seize 16-bit readings of 8 thermistors in lower than 1 second for $2

Determine 1 is an inexpensive and cheerful voltage inverter I provided awhile again in “A easy, correct, and environment friendly cost pump voltage inverter for $1”.

Determine 1 The generic and versatile xx4053 supplies the idea for an inexpensive, environment friendly, and correct voltage inverter.

Wow the engineering world together with your distinctive design: Design Concepts Submission Information

Shortly thereafter the concept morphed into an uncomplicated, cheap, and pretty quick (1 MHz), low energy voltage to frequency converter (VFC): “Voltage inverter design concept transmogrifies right into a 1 MHz VFC”. See Determine 2.

Determine 2 Modified voltage inverter turns into energy thrifty 1 MHz VFC.

 An attention-grabbing quirk of the Determine 2 circuit is that, though it’s described as a voltage to frequency converter, it might simply as precisely be referred to as a present to frequency converter. That’s as a result of enter present =Vin/R1 fees C3 making transconductance amplifier Q1, Q2 full a suggestions loop via the cost pump oscillator. The loop repeatedly adjusts Fpump to steadiness pump present to be equal and reverse to enter present,

Ipump = 5*C2*Fpump = Iin

Fpump = Iin / (5 C2 R1) = Iin / (5*100pF) = 2 kHz/µA

This makes doable some unusually easy and economical sensor interface topologies just like the one in Determine 3.

Determine 3 Eight thermistors can share a single excitation resistor.

A typical NTC thermistor’s datasheet (e.g., Molex 2152723605) summarizes its traits with 4 parameters like these from the 2152723605 sheet, proven in parentheses.

To = rated/calibration temperature (25°C = 298.15K)     (1)

Ro = resistance at To (10k+/-1%)                                       (2)

b = beta (3892K)                                                                   (3)

dissipation (self-heating) issue (1.5 mW/°C)                (4)

Then thermistor resistance (Rt) as a operate of temperature (T) in Kelvin is predicted by:

Rt = Ro exp(b(T^-1 – To^-1))                                                     (5)

 Selecting a price for excitation resistor R1 requires estimating the very best temperature (Tmax) any of the thermistors is predicted to be immersed in, and therefore the bottom resistance to be learn. Then we determine

Rx = Ro exp(b(Tmax^-1 – To^-1))                                             (6)

R1 = 10k – Rx                                                                        (7)

If calculation #7 requires R1 < 0, then R1 is in fact merely omitted. Taking for instance Tmax = 100^oC and numbers from the 2152723605 datasheet, Rx = 725, R1 = 10000 – 725 = 9275, to which the closest commonplace 1% worth is 9310.

The addressing of the thermistor to be learn is finished by output of the linked microcontroller by its three-bit binary U2 deal with on normal function output traces GP0-2. A microcontroller inner counter-timer peripheral linked to enter CTPin is then enabled to build up VFC pulses for two¹⁶ µs = 65.536 ms. Name the collected 16-bit integer ADC. Then the calculation of acquired temperatures, assuming 2152723605 sheet numbers once more, would proceed thusly:

X = ADC/2¹⁶ = 10k / (R1 + Rt)

X*(R1 + Rt) = 10k

Rt = 10k/X – R1 = 10k/X – 9310

T = (Ln(Rt/Rx)/b + Tmax^-1)^-1

^oOkay = (Ln(Rt/725))/3892 + 0.002680)^-1

^oC = Okay – 273.15

 The conversions are inherently radiometric, due to this fact insensitive to 5-V rail noise and tolerance, and integrating, making them extremely noise pickup resistant. Which aren’t unhealthy options for a two buck ADC.

Multiplexor U2’s error contributions—Ron and Roff—are, respectively, small (~60 ohms) and enormous (~100 Mohms) sufficient to not be vital elements.

Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Over 100 submissions have been accepted since his first contribution again in 1974.

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• Cheap VFC options good linearity and dynamic vary

The publish Seize 16-bit readings of 8 thermistors in lower than 1 second for $2 appeared first on EDN.

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