ADT7516 datasheet(21/44 Pages) AD

21 / 44 page

ADT7516/ADT7517/ADT7519

Rev. B | 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 of 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

selection is external reference.

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 (Bit 0 to Bit 3 of the DAC configuration

If a gain of 2 is selected (Bit 0 to Bit 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 18.

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 and the DAC B

output can be configured to represent the external temperature

sensor. Bit C5 and Bit 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 is 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 temperature

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

the DAC configuration register (Address 0x1B).

The output voltage is capable of tracking a maximum tempera-

ture 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, gives

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 0xD8 into the internal analog temperature offset

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

Use Equation 1 to Equation 4 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

UDB1

1.5

UDB1

+42

UDB1

+99

UDB1

2.25

UDB1

+127

UDB1

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

See Figure 41.

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–

N × I

IBIAS

Figure 45. Signal Conditioning for External Diode Temperature Sensor

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