AD9739-R2-EBZ datasheet(40/48 Pages) AD

40 / 48 page

AD9739

Data Sheet

Rev. B | Page 40 of 48

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

–15 –13 –11 –9 –7 –5 –3 –1 1

9 11 13 15

OFFSET CODE

CLKP

CLKN

Figure 61. Common-Mode Voltage with Respect to

CLKP_OFFSET/CLKN_OFFSET and DIR_P/DIR_N

VOLTAGE REFERENCE

The AD9739 output current is set by a combination of digital

control bits and the I120 reference current, as shown in Figure 62.

CURRENT

SCALING

FSC[9:0]

AD9739

DAC

IFULL-SCALE

10kΩ

1nF

VREF

I120

AGND

I120

VBG

1.2V

–

Figure 62. 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. Note the following

constraints when configuring the voltage reference circuit:

•

Both the 10 kΩ resistor and 1 nF bypass capacitor are required

for proper operation.

•

Adjusting the output full-scale current, IOUTFS, of the DAC

from its default setting of 20 mA should be performed

digitally.

•

The AD9739 is not a multiplying DAC. Modulating the

reference current, I120, with an ac signal is not supported.

•

The band gap voltage appearing at VREF (Pin C14) must

be buffered for use with external circuitry because its

output impedance is approximately 5 kΩ.

•

An external reference can be used to overdrive the internal

reference by connecting it to VREF (Pin C14).

IOUTFS can be adjusted digitally over 8.7 mA to 31.7 mA by using

FSC[9:0] (Register 0x06 and Register 0x07).

The following equation relates IOUTFS to the FSC[9:0] register,

which can be set from 0 to 1023:

IOUTFS = 22.6 × FSC[9:0]/1000 + 8.7

(1)

Note that a default value of 0x200 generates 20 mA full scale,

which is used for most of the characterization presented in this

data sheet (unless noted otherwise).

ANALOG OUTPUTS

Equivalent DAC Output and Transfer Function

The AD9739 provides complementary current outputs, IOUTP

and IOUTN, that source current into an external ground reference

load. Figure 63 shows an equivalent output circuit for the DAC.

Note that, compared to most current output DACs of this type,

the AD9739 outputs exhibit a slight offset current (that is,

IOUTFS/16), and the peak differential ac current is slightly below

IOUTFS/2 (that is, 15/32 × IOUTFS).

17/32 × IOUTFS

IPEAK =

15/32 × IOUTFS

70Ω

2.2pF

IOUTFS = 8.6 – 31.2mA

17/32 × IOUTFS

Figure 63. Equivalent DAC Output Circuit

As shown in Figure 63, the DAC output can be modeled as a pair of

dc current sources that source a current of 17/32 × IOUTFS to each

output. A differential ac current source, IPEAK, is used to model

the signal-dependent nature of the DAC output. The polarity

and signal dependency of this ac current source are related to

the digital code by the following equations:

F(Code) = (DACCODE − 8192)/8192

(2)

−1 ≤ F(Code) < 1

(3)

where DACCODE = 0 to 16,383 (decimal).

Because IPEAK can swing ±(15/32) × IOUTFS, the output currents

measured at IOUTP and IOUTN can span from IOUTFS/16 to

IOUTFS. However, because the ac signal-dependent current

component is complementary, the sum of the two outputs is

always constant (that is, IOUTP + IOUTN = (34/32) × IOUTFS).

The code-dependent current measured at the IOUTP (and

IOUTN) output is as follows:

IOUTP = 17/32 × IOUTFS + 15/32 × IOUTFS × F(Code)

(4)

IOUTN = 17/32 × IOUTFS − 15/32 × IOUTFS × F(Code)

(5)

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