AD9736BBC datasheet(48/72 Pages) AD

48 / 72 page

AD9734/AD9735/AD9736

Data Sheet

Rev. B | Page 48 of 72

ANALOG CONTROL REGISTERS

The AD973x includes some registers for optimizing its analog

performance. These registers include temperature trim for the

band gap, noise reduction in the output current mirror, and

output current mirror headroom adjustments.

BAND GAP TEMPERATURE CHARACTERISTIC

TRIM BITS

Using TRMBG<2:0> (Reg. 14, Bits 2:0), the temperature

characteristic of the internal band gap can be trimmed to

minimize the drift over temperature, as shown in Figure 86.

–50 –40 –30 –20 –10

1020

3040

506070

8090

TEMPERATURE (

C)

1.23

1.22

1.21

1.2

1.19

1.18

000

010

001

011

101

110

111

100

Figure 86. Band Gap Temperature Characteristic for Various TRMBG Values

The temperature changes are sensitive to process variations,

and Figure 86 may not be representative of all fabrication lots.

Optimum adjustment requires measurement of the device

operation at two temperatures and development of a trim

algorithm to program the correct TRMBG<2:0> values in

external nonvolatile memory.

MIRROR ROLL-OFF FREQUENCY CONTROL

With MSEL <1:0> (Reg. 14, Bits 7:6), the user can adjust the

noise contribution of the internal current mirror to optimize

the 1/f noise. Figure 87 shows MSEL vs. the 1/f noise with

20 mA full-scale current into a 50 Ω resistor.

F (kHz)

–110

–115

–120

–125

–130

–140

–135

100

MSEL0

MSEL2

MSEL1

MSEL3

Figure 87. 1/f Noise with Respect to MSEL Bits

HEADROOM BITS

HDRM<7:0> (Reg. 15, Bits 7:0) are for internal evaluation.

Changing the default reset values is not recommended.

VOLTAGE REFERENCE

The AD973x output current is set by a combination of digital

control bits and the I120 reference current, as shown in

Figure 88.

CURRENT

SCALING

FSC<9:0>

AD973x

DAC

IFULL-SCALE

10k

Ω

1nF

VREF

I120

AVSS

I120

VBG

1.2V

–

Figure 88. Voltage Reference Circuit

The reference current is obtained by forcing the band gap

voltage across an external 10 kΩ resistor from I120 (Pin B14) to

ground. The 1.2 V nominal band gap voltage (VREF) generates a

120 μA reference current in the 10 kΩ resistor. This current is

adjusted digitally by FSC<9:0> (Reg. 2, Reg. 3) to set the output

full-scale current IFS:





































1024

192

FSC

REF

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