INTERNAL REFERENCE OPERATION
The DC gain for this circuit is equal to feedback resistor RF.
At high frequencies, the DAC output impedance (CD1, CD2)
will produce a zero in the noise gain for the OPA2680 that
may cause peaking in the closed-loop frequency response.
CF is added across RF 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:
The DAC908 has an on-chip reference circuit that comprises
a 1.24V bandgap reference and a control amplifier. Ground-
ing pin 16, INT/EXT, enables the internal reference opera-
tion. The full-scale output current, IOUTFS, of the DAC908 is
determined by the reference voltage, VREF, and the value of
resistor RSET. IOUTFS can be calculated by:
IOUTFS = 32 • IREF = 32 • VREF / RSET
(10)
1
GBP
=
(8)
2πRFCF 4πRFCD
As shown in Figure 7, the external resistor RSET connects to
the FSA pin (Full-Scale Adjust). The reference control am-
plifier operates as a V-to-I converter producing a reference
current, IREF, which is determined by the ratio of VREF and
RSET, as shown in Equation 10. The full-scale output current,
IOUTFS, results from multiplying IREF by a fixed factor of 32.
with GBP = Gain Bandwidth Product of OPA
which will give a corner frequency f-3dB of approximately:
GBP
f−3dB
=
(9)
2πRFCD
+5V
CCOMPEXT
0.1µF
The full-scale output voltage is defined by the product of
IOUTFS • RF, and has a negative unipolar excursion. To
improve on the ac performance of this circuit, adjustment of
RF and/or IOUTFS 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.
BW
+VA
DAC908
VREF
RSET
IREF
=
FSA
Ref
Control
Amp
Current
Sources
REFIN
RSET
2kΩ
SINGLE-ENDED CONFIGURATION
CCOMP
400pF
0.1µF
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 IOUT, 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
0V to 0.5V into the 25Ω load.
INT/EXT
+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 con-
verter output can be adjusted from 20mA down to 2mA.
Operating the DAC908 at lower than 20mA output currents
may be desirable for reasons of reducing the total power
consumption, improving the distortion performance, or observ-
ing the output compliance voltage limitations for a given load
condition.
IOUTFS = 20mA
VOUT = 0V to +0.5V
IOUT
DAC908
IOUT
50Ω
50Ω
25Ω
It is recommended to bypass the REFIN pin with a ceramic chip
capacitor of 0.1µF or more. The control amplifier is internally
compensated, and its small signal bandwidth is approximately
1.3MHz. To improve the ac performance, an additional capaci-
tor (CCOMPEXT) should be applied between the BW pin and the
analog supply, +VA, 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 DAC908.
FIGURE 6. Driving a Doubly-Terminated 50Ω Cable Directly.
Different load resistor values may be selected as long as the
output compliance range is not exceeded. Additionally, the
output current, IOUTFS, and the load resistor may be mutually
adjusted to provide the desired output signal swing and
performance.
DAC908
SBAS109B
13
TI [ TEXAS INSTRUMENTS ]
