Datasheet

DAC904
13
SBAS095C
www.ti.com
FIGURE 5. Dual, Voltage-Feedback Amplifier OPA2680 Forms
Differential Transimpedance Amplifier.
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.
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
24ππRC
GBP
RC
FF FD
=
(8)
with GBP = Gain Bandwidth Product of OPA,
which will give a corner frequency f
-3dB
of approximately:
f
GBP
RC
dB
FD
−
=
3
2π
(9)
1/2
OPA2680
1/2
OPA2680
DAC904
–V
OUT
= I
OUT
• R
F
–V
OUT
= I
OUT
• R
F
R
F1
R
F2
C
F1
C
F2
C
D1
C
D2
I
OUT
I
OUT
50Ω
50Ω
–5V
+5V
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.
FIGURE 6. Driving a Doubly-Terminated 50Ω Cable Directly.
I
OUT
I
OUT
DAC904
25Ω
50Ω
50Ω
I
OUTFS
= 20mA
V
OUT
= 0V to +0.5V
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.