AD9736BBC datasheet(29/42 Pages) AD

29 / 42 page

Preliminary Technical Data

AD9736/AD9735/AD9734

Rev. PrJ | Page 29 of 42

VOLTAGE REFERENCE

The AD9736 output current is set by a combination of digital

control bits and the I120 reference current as shown in Figure 33.

Figure 33. Voltage Reference Circuit

The reference current is obtained by forcing the bandgap voltage

across an external 10kohm resistor from I120 (pin B14) to ground.

The 1.2V nominal bandgap voltage (Vref) will generate a 120uA

reference current in the 10k resistor. This current is adjusted

digitally by FSC<9:0> (REG02, REG03) to set the output full scale

current IFS:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛

1024

192

Vref

FSC

The full scale output current range is 10mA to 30mA for register

values from 0x000 to 0x3FF. The default value of 0x200 generates

20mA full scale. The typical range is shown in Figure 34.

200

400

600

800

1000

DAC gain code

Figure 34. IFS vs. DAC Gain Code

VREF (pin C14) must be bypassed to ground with a 1nF capacitor.

The bandgap voltage is present on this pin and may be buffered for

use in external circuitry. The typical output impedance is near

5kohms. If desired, an external reference may be used to overdrive

the internal reference by connecting it to the VREF pin.

IPTAT (pin D14) is used for factory testing. It may be left floating

(preferred) or tied to analog ground. It will output a current which

is proportional to absolute temperature. The nominal output is

approximately 10uA at 25C. The slope is approximately 20nA per

degree C.

Current

Scaling

DAC

Vbg

1.2V

FSC<9:0>

Ifull-scale

AD9736

I120

1nF

Vref

I120

10kΩ

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