ADT7516 datasheet(21/44 Pages) AD

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ADT7516/ADT7517/ADT7519

Rev. A | Page 21 of 44

DAC Reference Inputs

There is an input reference pin for the DACs. This reference

input is buffered (see Figure 44).

The advantage with the buffered input is the high impedance it

presents to the voltage source driving it. The user can have an

external reference voltage as low as 1 V and as high as VDD. The

restriction of 1 V is due to the footroom of the reference buffer.

The LDAC configuration register controls the option to select

between internal and external voltage references. The default

setting is for external reference selected.

Output Amplifier

The output buffer amplifier can generate output voltages to

within 1 mV of either rail. Its actual range depends on the value

of VREF, gain, and offset error.

If a gain of 1 is selected (Bits 0 to 3 of the DAC configuration

If a gain of 2 is selected (Bits 0 to 3 of the DAC configuration

of clamping, however, the maximum output is limited to

VDD – 0.001 V.

The output amplifier can drive a load of 4.7 kΩ to GND or VDD,

in parallel with 200 pF to GND or VDD (see Figure 5). The

source and sink capabilities of the output amplifier can be seen

in the plot of Figure 20.

The slew rate is 0.7 V/µs with a half-scale settling time to

±0.5 LSB (at 8 bits) of 6 µs.

Thermal Voltage Output

The ADT7516/ADT7517/ADT7519 can output voltages that are

proportional to temperature. DAC A output can be configured

to represent the temperature of the internal sensor while DAC B

output can be configured to represent the external temperature

sensor. Bits C5 and C6 of the Control Configuration 3 register

select the temperature proportional output voltage. Each time a

temperature measurement is taken, the DAC output is updated.

The output resolution for the ADT7519 is 8 bits with 1°C

change corresponding to 1 LSB change. The output resolution

for the ADT7516 and ADT7517 are capable of 10 bits with

0.25°C change corresponding to 1 LSB change. The default

output resolution for the ADT7516 and ADT7517 is 8 bits. To

increase this to 10 bits, set C1 = 1 in the Control Configuration

3 register. The default output range is 0 V to VREF and this can be

increased to 0 V to 2 VREF. Increasing the output voltage span to

2 VREF can be done by setting D0 = 1 for DAC A (internal temp-

erature sensor) and D1 = 1 for DAC B (external temperature

sensor) in the DAC configuration register (Address 1Bh).

The output voltage is capable of tracking a maximum temp-

erature range of –128°C to +127°C, but the default setting is

–40°C to +127°C. If the output voltage range is 0 V to VREF-IN

(VREF-IN = 2.25 V), then this corresponds to 0 V representing

–40°C, and 1.48 V representing +127°C. This, of course, will

give an upper deadband between 1.48 V and VREF.

The internal and external analog temperature offset registers

can be used to vary this upper deadband and, consequently, the

temperature that 0 V corresponds to. Table 6 and Table 7 give

examples of how this is done using a DAC output voltage span

of VREF and 2 VREF, respectively. Simply write in the temperature

value, in twos complement format, at which 0 V is to start. For

example, if using the DAC A output and 0 V to start at –40°C,

program D8h into the internal analog temperature offset reg-

ister (Address 21h). This is an 8-bit register and has a temp-

erature offset resolution of only 1°C for all device models. Use

the formulas following the tables to determine the value to

program into the offset registers.

Table 6. Thermal Voltage Output (0 V to VREF)

O/P Voltage (V)

Default °C

Max °C

Sample °C

–40

–128

0.5

+17

–71

+56

+73

–15

+113

1.12

+87

–1

+127

1.47

+127

+39

UDB

∗

1.5

UDB

∗

+42

UDB

∗

UDB

∗

+99

UDB

∗

2.25

UDB

∗

+127

UDB

∗

∗ Upper deadband has been reached. DAC output is not capable of

increasing. See Fig

ure 9

D+

LOW-PASS

FILTER

fC = 65kHz

BIAS

DIODE

VDD

TO ADC

VOUT+

VOUT–

REMOTE

SENSING

TRANSISTOR

(2N3906)

OPTIONAL CAPACITOR, UP TO

3nF MAX. CAN BE ADDED TO

IMPROVE HIGH FREQUENCY

NOISE REJECTION IN NOISY

ENVIRONMENTS

D–

× I

IBIAS

Figure 45. Signal Conditioning for External Diode Temperature Sensor

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