REV. B

–4–

AD22100

THEORY OF OPERATION

The AD22100 is a ratiometric temperature sensor IC whose

output voltage is proportional to power supply voltage. The

heart of the sensor is a proprietary temperature-dependent resis-

tor, similar to an RTD, which is built into the IC. Figure 4

shows a simplified block diagram of the AD22100.

V+

Ι

Figure 4. Simplified Block Diagram

change in resistance that is nearly linearly proportional to tem-

perature. This resistor is excited with a current source that is

proportional to power supply voltage. The resulting voltage

early varying with temperature. The remainder of the AD22100

consists of an op amp signal conditioning block that takes the

achieve the following output voltage function:

ABSOLUTE ACCURACY AND NONLINEARITY

SPECIFICATIONS

Figure 5 graphically depicts the guaranteed limits of accuracy

for the AD22100 and shows the performance of a typical part.

As the output is very linear, the major sources of error are offset,

i.e., error at room temperature, and span error, i.e., deviation

from the theoretical 22.5 mV/

°C. Demanding applications can

achieve improved performance by calibrating these offset and

gain errors so that only the residual nonlinearity remains as a

significant source of error.

TEMPERATURE –

°C

4

–4

150

–2

–3

–50

0

–1

1

2

3

100

50

0

MAXIMUM ERROR

OVER TEMPERATURE

TYPICAL ERROR

MAXIMUM ERROR

OVER TEMPERATURE

Figure 5. Typical AD22100 Performance

OUTPUT STAGE CONSIDERATIONS

As previously stated, the AD22100 is a voltage output device. A

basic understanding of the nature of its output stage is useful for

proper application. Note that at the nominal supply voltage of

5.0 V, the output voltage extends from 0.25 V at –50

°C to

+4.75 V at +150

°C. Furthermore, the AD22100 output pin is

capable of withstanding an indefinite short circuit to either

ground or the power supply. These characteristics are provided

by the output stage structure shown in Figure 6.

V+

Ι

Figure 6. Output Stage Structure

The active portion of the output stage is a PNP transistor with

its emitter connected to the V+ supply and collector connected

to the output node. This PNP transistor sources the required

amount of output current. A limited pull-down capability is

provided by a fixed current sink of about –80

µA. (Here,

“fixed” means the current sink is fairly insensitive to either sup-

ply voltage or output loading conditions. The current sink ca-

pability is a function of temperature, increasing its pull-down

capability at lower temperatures.)

Due to its limited current sinking ability, the AD22100 is inca-

pable of driving loads to the V+ power supply and is instead in-

tended to drive grounded loads. A typical value for short circuit

current limit is 7 mA, so devices can reliably source 1 mA or

2 mA. However, for best output voltage accuracy and minimal

internal self-heating, output current should be kept below 1 mA.

Loads connected to the V+ power supply should be avoided as

the current sinking capability of the AD22100 is fairly limited.

These considerations are typically not a problem when driving

a microcontroller analog to digital converter input pin (see

MICROPROCESSOR A/D INTERFACE ISSUES).

RATIOMETRICITY CONSIDERATIONS

The AD22100 will operate with slightly better accuracy than

that listed in the data sheet specifications if the power supply is

held constant. This is because the AD22100’s output voltage

varies with both temperature and supply voltage, with some

errors. The ideal transfer function describing the output

voltage is:

(V+/5 V)

× [1.375 V + (22.5 mV/°C) × T

The ratiometricity error is defined as the percent change away

from the ideal transfer function as the power supply voltage

changes within the operating range of +4 V to +6 V. For the

AD22100 this error is typically less than 1%. A movement from

the ideal transfer function by 1% at +25

°C, with a supply volt-

age varying from 5.0 V to 5.50 V, results in a 1.94 mV change in

output voltage or 0.08

°C error. This error term is greater at

higher temperatures because the output (and error term) is di-

rectly proportional to temperature. At 150

°C, the error in out-

put voltage is 4.75 mV or 0.19

°C.