DUAL TRANSIMPEDANCE OUTPUT CONFIGURATION
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.
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.
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 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.
+5V
50Ω
1/2
OPA2680
–VOUT = IOUT • RF
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.
RF1
DAC904
CF1
IOUT
CD1
RF2
CF2
IOUTFS = 20mA
VOUT = 0V to +0.5V
CD2
IOUT
IOUT
DAC904
1/2
OPA2680
IOUT
50Ω
50Ω
–VOUT = IOUT • RF
25Ω
50Ω
–5V
FIGURE 6. Driving a Doubly-Terminated 50Ω Cable Directly.
FIGURE5.Dual,Voltage-FeedbackAmplifierOPA2680Forms
Differential Transimpedance Amplifier.
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, IOUTFS, of the DAC904 is
determined by the reference voltage, VREF, and the value of
resistor RSET. IOUTFS can be calculated by:
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:
IOUTFS = 32 • IREF = 32 • VREF / RSET
(10)
The external resistor RSET 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,
1
GBP
=
(8)
2πRFCF 4πRFCD
with GBP = Gain Bandwidth Product of OPA,
I
REF, which is determined by the ratio of VREF and RSET, as
shown in Equation 10. The full-scale output current, IOUTFS
,
which will give a corner frequency f-3dB of approximately:
results from multiplying IREF by a fixed factor of 32.
GBP
f−3dB
=
(9)
2πRFCD
DAC904
SBAS095C
13
www.ti.com
TI [ TEXAS INSTRUMENTS ]
