AD-DPGIOZ datasheet(41/61 Pages) AD

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Data Sheet

AD9961/AD9963

Rev. A | Page 41 of 60

Transmit Path Gain Adjustment

Adjusting the output signal level is implemented by scaling the

full-scale output current of the transmit DAC. There are four

separate programmable parameters that can be used to adjust

the full-scale output of the DACs; the REFIO voltage, the RSET

resistance, and the fine and coarse gain control parameters.

Adjusting the REFIO Voltage

There is a single reference voltage that is used by both the I and

Q channel DACs. The REFIO reference voltage is generated by

an internal 100 µA current source terminated into a programmable

resistor, RREF. The nominal RREF resistance is 10 kΩ resulting in a

1.0 V reference voltage. The resistance can be varied by adjusting

the REFIO_ADJ[5:0] bits in Register 0x6E. This adds or subtracts

up to 20% from the RREF resistance and hence the REFIO voltage

and the DAC full-scale current. A secondary effect to changing

the REFIO voltage is that the full-scale voltage in the auxiliary

DACs also changes by the same magnitude.

The register uses twos complement format in which 011111

maximizes the voltage on the REFIO node and 100000

minimizes the voltage. A curve illustrating the variation of

REFIO voltage vs. REFIO_ADJ value is shown in Figure 54.

0.7

0.8

0.9

1.0

1.1

1.2

1.3

REFIO ADJ

Figure 54. Typical VREFIO Voltage vs. REFIO_ADJ Value

The REFIO pin should be decoupled to AGND with a 0.1 µF

capacitor. If the voltage at REFIO is to be used for external

purposes, an external buffer amplifier with an input bias current

of less than 100 nA should be used.

An external reference can be used in applications requiring

tighter gain tolerances or lower temperature drift. Also, a variable

external voltage reference can be used to implement a method

for gain control of the DAC output. The external reference is

applied to the REFIO pin. Note that the 0.1 µF compensation

capacitor is not required. The internal reference can be directly

overdriven by the external reference, or the internal reference

can be powered down. The input impedance of REFIO is 10 kΩ

when powered up and 1 MΩ when powered down.

Table 20. Reference Operation

Reference

Mode

REFIO Pin

Internal

Connect 0.1 μF capacitor

(default)

External

Apply external reference

(disables internal

reference)

Adjusting the Current Scaling Resistor

Each transmit DAC has a resistor that is used to adjust the full-

scale current. The nominal resistance is 16 kΩ, which results in

a full-scale current of 2 mA (when VREFIO equals 1.0 V). The

6-bit programmable values, IRSET[5:0] and QRSET[5:0]

(Register 0x6A and Register 0x6D), provide an output current

adjustment range of ±20% as shown in Figure 55.

1.6

1.8

2.0

2.2

2.4

2.6

RSET (Ω)

Figure 55. Output Current Scaling vs. IRSET and QRSET Values

Adjusting the GAIN Parameters

Each transmit DAC has coarse and fine gain control parameters

for scaling the full-scale output currents. These adjustments

change only the full-scale current of the DAC and have no

impact on the REFIO voltage. The coarse scale adjust (GAIN1)

allows the nominal output current to be changed by ±6 dB in

approximately 0.25 dB steps. The adjustment range of the fine

scale adjust (GAIN2) is about ±2.5%. Figure 56 and Figure 57

show the resulting gain scaling vs. the GAIN1 and GAIN2

parameters.

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