ADT14

–11–

REV. 0

Buffering the Temperature Output Pin

The VPTAT sensor output is a low impedance dc output volt-

age with a 5 mV/K temperature coefficient, and is useful in a

number of measurement and control applications. In many

applications, this voltage may need to be transmitted to a central

location for processing. The unbuffered VPTAT voltage output

is capable of 500

µA drive into 50 pF (max). As mentioned in

is useful to consider external amplifiers for interfacing VPTAT

to external circuitry to ensure accuracy, and to minimize load-

ing, which could create dissipation-induced temperature sensing

errors. An excellent general-purpose buffer circuit using the

OP177, which is capable of driving over 10 mA and will remain

stable under capacitive loads of up to 0.1

µF, is shown in Figure

20. Other interface circuits are shown below.

V+

V–

0.1 F

10k

Ω

100

Ω

OP177

ADT14

VPTAT

Figure 20. Buffer VPTAT to Handle Difficult Loads

Differential Transmitter

In noisy industrial environments, it is difficult to send an accu-

rate analog signal over a significant distance. However, by send-

ing the signal differentially on a wire pair, these errors can be

significantly reduced. Since the noise will be picked up equally

on both wires, a receiver with high common-mode input rejec-

tion can be used very effectively to cancel out the noise at the

receiving end. Figure 21 shows two amplifiers being used to

send the signal differentially, and an excellent differential re-

ceiver, the AMP03, (SSM2141 or SSM2143 are two other

options), which features a common-mode rejection ratio of

95 dB at dc and very low input and drift errors.

V+

V–

50

Ω

1/2 OP297

ADT14

VPTAT

10k

Ω

4.9k

Ω

10k

Ω

1/2 OP297

50

Ω

V+

V–

AMP03 OR

SSM2143

Figure 21. Send the Signal Differentially for Noise

Immunity

4 mA to 20 mA Current Loop

Another very common method of transmitting a signal over long

distances is to use a 4 mA-to-20 mA loop (see Figure 22). An

advantage of using a 4 mA-to-20 mA loop is that the accuracy of

a current loop is not compromised by voltage drops across the

line. One requirement of 4 mA-to-20 mA circuits is that the

remote end must receive all of its power from the loop, meaning

that the circuit must consume less than 4 mA. Operating from

+5 V, the quiescent current of the ADT14 is 600

µA max, and

the OP90s is 20

µA max, totaling much less than 4 mA. Although

not shown, the open collector outputs and temperature setting

pins can be connected to do any local control of switching.

The current is proportional to the voltage on the VPTAT out-

put, and is calibrated to 4 mA at a temperature of –40

°C, to

20 mA for +85

°C. The main equation governing the operation

of this circuit gives the current as a function of VPTAT:

1

R6

VPTAT

× R5

R2









−

R3

+ R1

×1+

R5

R2









ADT14

VPTAT

6

7

2

3

4

V+

GND

OP90

R2

39.2k

Ω

R1

243k

Ω

R3

100k

Ω

R6

100

Ω

R5

100k

Ω

2N1711

14

5

13

6

+5V TO +13.2V

4-20mA

Figure 22. 4 mA to 20 mA Current Loop

To determine the resistor values in this circuit, first note that

VPTAT varies from 1.165 V at –40

°C to 1.79 V at +85°C. The

absolute value of the resistors is not important, only the ratio.

For convenience, 100 k

Ω is chosen for R5. Once R2 is calcu-

lated, the value of R3 and R1 is determined by substituting

values are shown in the circuit. The OP90 is chosen for this

circuit because of its ability to operate on a single supply and its

high accuracy. For initial accuracy, a 10 k

Ω trim potentiometer

can be included in series with R3, and the value of R3 lowered

to 95 k

Ω. The potentiometer should be adjusted to produce an

output current of 12.3 mA at 25

°C.

Temperature-to-Frequency Converter

Another common method of transmitting analog information is

to convert a voltage to the frequency domain. This is easily done

with any of the available low cost monolithic Voltage-to-Fre-

quency Converters (VFCs) that feature an open-collector digital

output. A digital signal is immune to noise and voltage drops

because the only important information is the frequency. As

long as the conversions between temperature and frequency are

accurately performed, the temperature data can be accurately

transmitted.

A simple circuit to do this combines the ADT14 with an AD654

VFC and is shown in Figure 23. The AD654 outputs a square

wave that is proportional to the dc input voltage according to

the following equation:

10 (R1

+ R2)C

T