AD7294BCPZRL datasheet(26/48 Pages) AD

26 / 48 page

Data Sheet

AD7294

Rev. I | Page 25 of 47

Series Resistance Cancellation

The AD7294 has been designed to automatically cancel out the

effect of parasitic, base, and collector resistance on the tempera-

ture reading. This gives a more accurate result, without the need

for any user characterization of the parasitic resistance. The

AD7294 can compensate for up to 100 Ω in a process that is

transparent to the user.

DAC OPERATION

The AD7294 contains four 12-bit DACs that provide digital

control with 12 bits of resolution with a 2.5 V internal reference.

The DAC core is a thin film 12-bit string DAC with a 5 V output

span and an output buffer that can drive the high voltage output

stage. The DAC has a span of 0 V to 5 V with a 2.5 V reference

input. The output range of the DAC, which is controlled by the

offset input, can be positioned from 0 V to 15 V.

Resistor String

The resistor string structure is shown in Figure 48. It consists of

a string of 2n resistors, each of Value R. The code loaded to the

DAC register determines at which node on the string the voltage

is tapped off to be fed into the output amplifier. The voltage is

tapped off by closing one of the switches connecting the string

to the amplifier. This architecture is inherently monotonic,

voltage out, and low glitch. It is also linear because all of the

resistors are of equal value.

TO OUTPUT

AMPLIFIER

Figure 48. Resistor String Structure

Output Amplifier

Referring to Figure 48, the purpose of A1 is to buffer the DAC

output range from 0 V to VREF. The second amplifier, A2, is

configured such that when an offset is applied to OFFSET IN x,

its output voltage is three times the offset voltage minus twice

the DAC voltage.

VOUT = 3VOFFSET − 2VDAC

The DAC word is digitally inverted on-chip such that

VOUT = 3VOFFSET + 2(VDAC − VREF)

and VDAC =

























REF

where:

VDAC is the output of the DAC before digital inversion.

D is the decimal equivalent of the binary code that is loaded to the

DAC register.

n is the bit resolution of the DAC.

An example of the offset function is given in Table 8.

Table 8. Offset Voltage Function Example

Offset

Voltage

VOUT with 0x000

VOUT with 0xFFF

1.67 V

0 V

5 V − 1 LSB

3.33 V

5 V

10 V − 1 LSB

5.00 V

10 V

15 V − 1 LSB

The user has the option of leaving the offset pin open, in which

case the voltage on the noninverting input of Op Amp A2 is set

by the resistor divider, giving

VOUT = 2VDAC

This generates the 5 V output span from a 2.5 V reference.

Digitally inverting the DAC allows the circuit to operate as a

generic DAC when no offset is applied. If the offset pin is not

being driven, it is best practice to place a 100 nF capacitor

between the pin and ground to improve both the settling time

and the noise performance of the DAC.

Note that a significant amount of power can be dissipated in the

DAC outputs. A thermal shutdown circuit sets the DAC outputs

to high impedance if a die temperature of >150°C is measured

by the internal temperature sensor. This also sets the overtem-

perature alert bit in Alert Register C, see the Alerts and Limits

Theory section. Note that this feature is disabled when the

temperature sensor powers down.

ADC AND DAC REFERENCE

The AD7294 has two independent internal high performance

2.5 V references, one for the ADCs and the other for the four

on-chip DACs. If the application requires an external reference,

it can be applied to the REFOUT/REFIN DAC pin and/or to the

REFOUT/REFIN ADC pin. The internal reference should be buffered

before being used by external circuitry. Decouple both the REFOUT/

REFIN DAC pin and the REFOUT/REFIN ADC pin to AGND using a

220 nF capacitor. On power-up, the AD7294 is configured for

use with an external reference. To enable the internal references,

write a zero to both the D4 and D5 bits in the power-down

the ADC and DAC references require a minimum of 60 μs to

power up and settle to a 12-bit performance when a 220 nF

decoupling capacitor is used.

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