DAC900-Q1_15 Datasheet, PDF (19/29 Pages) Texas Instruments – 10-BIT 165-MSPS DIGITAL-TO-ANALOG CONVERTER

DAC900-Q1
www.ti.com
SBAS505 – JUNE 2010
Internal Reference Operation
The DAC900 has an on-chip reference circuit that comprises a 1.24V bandgap reference and a control amplifier.
Grounding pin 16, INT/EXT, enables the internal reference operation. The full-scale output current, I_OUTFS, of the
DAC900 is determined by the reference voltage, V_REF, and the value of resistor R_SET. I_OUTFS can be calculated
by:
I_OUTFS
= 32 × I_REF
= 32 ×
V_REF
R_SET
(10)
As shown in Figure 7, the external resistor RSET connects to the FSA pin (Full-Scale Adjust). The reference
control amplifier operates as a V-to-I converter producing a reference current, I_REF, which is determined by the
ratio of V_REF and R_SET, as shown in Equation 10. The full-scale output current, I_OUTFS, results from multiplying
I_REF by a fixed factor of 32.
C_COMPEXT ^+5V
0.1µF
I_REF
=
V_REF
R_SET
DAC900
FSA
R_SET
2kΩ
REF_IN
0.1µF
INT/EXT
Ref
Control
Amp
BW
+V_A
Current
Sources
C_COMP
400pF
+1.24V Ref.
Figure 7. Internal Reference Configuration
Using the internal reference, a 2kΩ resistor value results in a 20mA full-scale output. Resistors with a tolerance
of 1% or better should be considered. Selecting higher values, the converter output can be adjusted from 20mA
down to 2mA. Operating the DAC900 at lower than 20mA output currents may be desirable for reasons of
reducing the total power consumption, improving the distortion performance, or observing the output compliance
voltage limitations for a given load condition.
It is recommended to bypass the REF_IN pin with a ceramic chip capacitor of 0.1mF or more. The control amplifier
is internally compensated, and its small signal bandwidth is approximately 1.3MHz. To improve the ac
performance, an additional capacitor (C_COMPEXT) should be applied between the BW pin and the analog supply,
+V_A, as shown in Figure 7. Using a 0.1mF capacitor, the small-signal bandwidth and output impedance of the
control amplifier is further diminished, reducing the noise that is fed into the current source array. This also helps
shunting feedthrough signals more effectively, and improving the noise performance of the DAC900.
Copyright © 2010, Texas Instruments Incorporated
Product Folder Link(s): DAC900-Q1
Submit Documentation Feedback
19