AD7294 datasheet(19/45 Pages) AD

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Preliminary Technical Data

AD7294

Rev. PrB | Page 19 of 45

The AD7294 automatically cancels out the effect of this series

resistance on the temperature reading, giving a more accurate

result, without the need for user characterization of this resis-

tance. The AD7294 is designed to automatically cancel typically

up to 3 kΩ of resistance. By using an advanced temperature

measurement method, this is transparent to the user. This

feature allows resistances to be added to the sensor path to

produce a filter, allowing the part to be used in noisy environ-

ments.

Temperature Measurement Method

The temperature reading from the ADC is stored in a 10-bit

twos complement format and in a sign bit format, D10 to D0, to

accommodate both positive and negative temperature measure-

ments. The temperature data format is shown in Table 6, which

shows the achievable temperature measurement range. The

ADC can theoretically measure a 512°C temperature span,

ranging from −256°C to +255.75°C with an LSB of 0.25°C.

Table 6. TSENSE Data Format

Digital Input

Bit Weight (°C)

111 1111 1111

−0.25

100 0000 0000

−256

011 1111 1111

+255.75

000 0000 0000

+0.0

Each input is integrated in turn over a period of several hundred

microseconds. This takes place continuously in the background,

leaving the user free to perform conversions on the other channels.

When integration is complete, a signal is passed to the control

logic to automatically initiate a conversion. If the ADC is in

command mode, the temperature conversion is performed as

soon as the next conversion is completed. In autocycle mode,

the conversion is inserted into an appropriate place in the current

sequence; see the Register Setting section for further details. If

the ADC is idle, the conversion takes place immediately.

Three registers store the result of the last conversion on each

temperature channel; these can be read at any time. In addition,

in command mode, one or both of the two external channel

registers can be read out as part of the output sequence. The

MSB of the registers is set if an open-circuit condition is

detected on the input of the external sensors, indicating an

invalid result.

Noise Filtering

For temperature sensors operating in noisy environments,

previous practice was to place a capacitor across the D+ pin and

D− pin to help combat the effects of noise. However, large capaci-

tances affect the accuracy of the temperature measurement,

leading to a recommended maximum capacitor value of 1000 pF.

This capacitor reduces the noise, but does not eliminate it.

Sometimes, this sensor noise is a problem in a very noisy

environment. In most cases, a capacitor is not required as

differential inputs, by their very nature, have a high immunity

to noise.

Remote Sensing Diode

To reduce the error due to variations in both substrate and

discrete transistors, a number of factors should be taken into

consideration:

•

The ideality factor, nf, of the transistor is a measure of the

deviation of the thermal diode from ideal behavior. The

AD7294 is trimmed for an nf value of 1.008. Use the

following equation to calculate the error introduced at a

temperature T (°C), when using a transistor whose nf

does not equal 1.008. See the processor data sheet for the

nf values.

∆T = (nf − 1.008) × (273.15 K + T)

To factor this in, the user can write the ∆T value to the offset

subtracts it from the temperature measurement.

If a discrete transistor is used with the AD7294, the best

accuracy is obtained by choosing devices according to the

following criteria:

•

Base-emitter voltage greater than 0.25 V at 11 µA, at the

highest operating temperature.

•

Base-emitter voltage less than 0.95 V at 180 µA, at the

lowest operating temperature.

•

Base resistance less than 100 Ω.

•

Small variation in hFE (approximately 50 to 150) that

indicates tight control of VBE characteristics.

REFERENCE FOR ADC/DAC

The internal reference on the AD7294 is a well regulated, low

ppm 2.5 V reference with low drift voltage and a stable output.

The reference is common to all four DAC channels and the SAR

ADC. If the application requires an external reference, it can be

applied to the REFOUT/REFIN DAC pin or to the REFOUT/

REFIN ADC pin. If the reference is driving external nodes, a

buffer is required to achieve a low impedance output. For

stability, the reference buffers each require at least 470 nF on the

output pin. Note that on power up, the ADC and DAC reference

buffers are switched off by default ; note the power-down

When using an external reference to achieve the desired

performance from the AD7294, thought should be given to the

choice of a precision voltage reference. There are four possible

sources of error that should be considered when choosing a

voltage reference for high accuracy applications: initial

accuracy, ppm drift, long-term drift, and output voltage noise.

To minimize these errors, a reference with high initial accuracy

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