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Data Sheet

AD9119/AD9129

Rev. B | Page 51 of 66

VOLTAGE REFERENCE

The AD9119/AD9129 output current is set by a combination of

digital control bits and the I250U reference current, as shown in

Figure 149.

CURRENT

SCALING

FSC[9:0]

AD9129

DAC

IFULLSCALE

Ω

1nF

VREF

I250

–1.5V

I250U

VBG

1.0V

–

Figure 149. Voltage Reference Circuit

The reference current is obtained by forcing the band gap

voltage across an external 4.0 kΩ resistor from I250U (Pin A1)

to VSSA. The 1.0 V nominal band gap voltage (VREF) generates a

250 µA reference current in the 4.0 kΩ resistor. Note the

following constraints when configuring the voltage reference

circuit:

•

Both the 4.0 kΩ resistor and 1 nF bypass capacitor are

required for proper operation.

•

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

its default setting of 20 mA should be performed digitally.

•

The AD9119/AD9129 are not multiplying DACs. Modula-

tion of the reference current, I250U, with an ac signal is not

supported.

•

The band gap voltage appearing at the VREF pin must be

buffered for use with an external circuitry because its output

impedance is approximately 7.5 kΩ.

•

An external reference can be used to overdrive the internal

reference by connecting it to the VREF pin.

As mentioned, the IOUTFS can be adjusted digitally over a 9.4 mA

to 34.2 mA range by the FSC_x[9:0] bits (Register 0x20, Bits[7:0]

and Register 0x21, Bits[1:0]). The following equation relates

IOUTFS to the FSC_x[9:0] bits, which can be set from 0 to 1023.

IOUTFS = 24.21875 mA × FSC_x[9:0]/1000 + 9.4 mA

(1)

Note that the default value of 0x200 generates 21.937 mA full scale,

but most of the characterization presented in this datasheet uses

33 mA, unless noted otherwise.

ANALOG OUTPUTS

Equivalent DAC Output and Transfer Function

The AD9119/AD9129 provide complementary current outputs,

IOUTP and IOUTN, that sink current from an external load

that is referenced to the 1.8 V VDDA supply. Figure 150 shows

an equivalent output circuit for the DAC. Compared to most

current output DACs of this type, the outputs of the AD9119/

AD9129 exhibit a slight offset current (that is, IOUTFS/17), and the

peak differential ac current is slightly below IOUTFS/2 (that is,

8/17 × IOUTFS).

(9/17) × IOUTFS

IPEAK =

(8/17) × IOUTFS

IOUTFS = 9.5mA – 34mA

(9/17) × IOUTFS

Figure 150. Equivalent DAC Output Circuit

The example shown in Figure 150 can be modeled as a pair of

dc current sources that source a current of 9/17 × IOUTFS to each

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

the signal (that is, a digital code) dependent nature of the DAC

output. The polarity and signal dependency of this ac current

source is related to the digital code (F) by the following equation:

F (code) = (DACCODE − 8192)/8192

(2)

−1 < F (code) < +1

(3)

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

Because IPEAK can swing ±(8/17) × IOUTFS, the output currents

that are measured at IOUTP and IOUTN can span from

IOUTFS/17 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 =

(18/17) × IOUTFS).

The code-dependent current that is measured at the IOUTP

(and IOUTN) output is as follows:

IOUTP = (9/17) × IOUTFS

(mA) + (8/17) × IOUTFS (mA)

× F (code)

(4)

IOUTN = (9/17) × IOUTFS

(mA) − (8/17) × IOUTFS (mA)

× F (code)

Figure 151 shows the IOUTP vs. DACCODE transfer function

when IOUTFS is set to 19.65 mA.

4096

8192

12,288

DAC CODE

16,384

Figure 151. Gain Curve for FSC_x[9:0] = 512, DAC Offset = 1.228 mA

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