DAC904 Datasheet, PDF (13/23 Pages) Burr-Brown (TI) – 14-Bit, 165MSPS DIGITAL-TO-ANALOG CONVERTER

DUAL TRANSIMPEDANCE OUTPUT CONFIGURATION
The circuit example of Figure 5 shows the signal output
currents connected into the summing junction of the OPA2680,
which is set up as a transimpedance stage, or I-to-V con-
verter. With this circuit, the DAC’s output will be kept at a
virtual ground, minimizing the effects of output impedance
variations, and resulting in the best DC linearity (INL). How-
ever, as mentioned previously, the amplifier may be driven
into slew-rate limitations, and produce unwanted distortion.
This may occur especially at high DAC update rates.
+5V
50Ω
1/2
OPA2680
DAC904
I_OUT
C_D1
I_OUT
C_D2
R_F1
C_F1
R_F2
C_F2
1/2
OPA2680
50Ω
–5V
–V_OUT = I_OUT • R_F
–V_OUT = I_OUT • R_F
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
0V to 0.5V into the 25Ω load.
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 mutually
adjusted to provide the desired output signal swing and
performance.
I_OUTFS = 20mA
I_OUT
DAC904
I_OUT
25Ω
50Ω
V_OUT = 0V to +0.5V
50Ω
FIGURE 5. Dual, Voltage-Feedback Amplifier OPA2680 Forms
Differential Transimpedance Amplifier.
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:
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
2πR_FC_D
(9)
FIGURE 6. Driving a Doubly-Terminated 50Ω Cable Directly.
INTERNAL REFERENCE OPERATION
The DAC904 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, I_OUTFS, of the DAC904 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)
The external resistor R_SET connects to the FSA pin (Full-
Scale Adjust), see Figure 7. 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.
DAC904
13
SBAS095C
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