DAC900 Datasheet, PDF (14/16 Pages) Burr-Brown (TI) – 10-Bit, 165MSPS DIGITAL-TO-ANALOG CONVERTER

The DC gain for this circuit is equal to feedback resistor R_F.
At high frequencies, the DAC output impedance (C_D1, C_D2)
will produce a zero in the noise gain for the OPA2680 that
may cause peaking in the closed-loop frequency response.
C_F is added across R_F to compensate for this noise gain
peaking. To achieve a flat transimpedance frequency re-
sponse, the pole in each feedback network should be set to:
INTERNAL REFERENCE OPERATION
The DAC900 has an on-chip reference circuit which com-
prises a 1.24V bandgap reference and a control amplifier.
Grounding of pin 16, INT/EXT, enables the internal refer-
ence 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:
1 ₌ GBP
2πR_FC_F 4πR_FC_D
(8)
with GBP = Gain Bandwidth Product of OPA
which will give a corner frequency f_-3dB of approximately:
f_−3dB
=
GBP
2 πR _F C _D
(9)
The full-scale output voltage is defined by the product of
I_OUTFS • R_F, and has a negative unipolar excursion. To
improve on the ac performance of this circuit, adjustment of
R_F and/or I_OUTFS should be considered. Further extensions of
this application example may include adding a differential
filter at the OPA2680’s output followed by a transformer, in
order to convert to a single-ended signal.
SINGLE-ENDED CONFIGURATION
Using a single load resistor connected to the one of the DAC
outputs, a simple current-to-voltage conversion can be ac-
complished. The circuit in Figure 6 shows a 50Ω resistor
connected to I_OUT, providing the termination of the further
connected 50Ω cable. Therefore, with a nominal output
current of 20mA, the DAC produces a total signal swing of
0 to 0.5V into the 25Ω load.
I_OUTFS = 32 • I_REF = 32 • V_REF / R_SET
(10)
As shown in Figure 7, the external resistor R_SET connects to
the FSA pin (Full-Scale Adjust). The reference control
amplifier operates as a V to I converter producing a refer-
ence current, I_REF, which is determined by the ratio of V_REF
and R_SET (see 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.
I_OUTFS = 20mA
I_OUT
DAC900
I_OUT
25Ω
50Ω
V_OUT = 0V to +0.5V
50Ω
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 ob-
serving the output compliance voltage limitations for a given
load condition.
FIGURE 6. Driving a Doubly Terminated 50Ω Cable Directly.
It is recommended to bypass the REF_IN pin with a ceramic chip
capacitor of 0.1µF or more. The control amplifier is internally
compensated, and its small signal bandwidth is approximately
Different load resistor values may be selected as long as the
output compliance range is not exceeded. Additionally, the
output current, I_OUTFS, and the load resistor, may be mutu-
ally adjusted to provide the desired output signal swing and
performance.
1.3MHz. To improve the ac performance, an additional capaci-
tor (C_COMPEXT) should be applied between the BW pin and the
analog supply, +V_A, as shown in Figure 7. Using a 0.1µF
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.
®
DAC900
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