AD9961-EBZ(Rev0) View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
AD9961-EBZ Datasheet PDF : 60 Pages
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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.
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0
8
16
24
32
40
48
56
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.
AD9961/AD9963
Table 20. Reference Operation
Reference
Mode
REFIO Pin
Internal Connect 0.1 μF capacitor
External Apply external reference
Register Setting
Register 0x60, Bit 6 = 0
(default)
Register 0x60, Bit 6 = 1
(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.
2.6
2.4
2.2
2.0
1.8
1.6
0
8
16
24
32
40
48
56
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.
Rev. 0 | Page 41 of 60
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