Datasheet

1
2pR
F
C
F
+
GBP
4pR
F
C
D
Ǹ
f
*3dB
+
GBP
2pR
F
C
D
Ǹ
OPA890
www.ti.com SBOS369B –MAY 2007–REVISED DECEMBER 2009
Driving a light load, the OPA890 can output ±4V over Notice that most of the error occurs mainly at the first
±5V supplies. Setting the reference voltage to –5V codes (0, 1, 2); excluding these codes from the
results in an output voltage swing from 0V to 5V. In analysis yields the following results, shown in
order to optimize the OPA2890 operation for this Table 1.
application, the supply voltages have been adjusted
so that the output voltage swing is balanced around Table 1. DC Accuracy vs Code
mid-supply of the amplifier. Note that as a result of
TOTAL ERROR DUE TO
the internal architecture of the multiplying DAC, the
CODES V
OS
and I
B
I
OUT1
output is not high impedance. The I
OUT1
output
All codes 3.9LSB
resistance is between 4.5kΩ and 22.1kΩ (excluding
Excluding code 0 2.5LSB
code 000h) for a 10kΩ nominal V
REF
input resistance.
Excluding codes 0 and 1 2LSB
I
OUT1
output resistance changes are directly related to
the code change. This low impedance has multiple
Excluding codes 0, 1, and 2 1.83LSB
effects when a bipolar technology amplifier is used.
Note that 1LSB = 1.221mV in the example shown in
Some of these effects are:
Figure 48
• The noise gain of the amplifier changes for each
If more precision is required while maintaining the ac
code.
performance, a FET-input amplifier (such as the
• The output offset voltage of the amplifier changes
OPA656 or the THS4631) is a good alternative.
for each code, because of the input offset voltage.
Figure 48 shows a single-ended output drive
• The input bias current cannot be cancelled. The
implementation. In this circuit, only one side of the
effects of the input bias current can be reduced,
complementary output drive signal is used. A dual
but not eliminated, thereby affecting the total
amplifier, such as the OPA2890, provides both output
output offset voltage of the amplifier with each
drivers for the DAC7822. If even lower quiescent
code.
current is needed, the OPA2889 can be used instead,
• The noninverting pin of the amplifier must be tied
with minor modifications. The diagram shows the
to ground and cannot be used to create a dc
signal output current connected into the virtual ground
offset on the output amplifier, as is the case for
summing junction of the OPA890, which is set up as
the transimpedance amplifier.
a transimpedance stage or I-V converter. The unused
The following analysis excludes the input offset
current output of the DAC is connected to ground.
current.
The dc gain for this circuit is equal to R
F
. At high
frequencies, the DAC output capacitance produces a
The total output offset voltage variations because of
zero in the noise gain for the OPA890 that may cause
code changing in the DAC can be expressed as:
peaking in the closed-loop frequency response. C
F
is
ΔV
OSO
= +ΔNG {[(R
F
R
OUT1
) – R
S
] + V
O
S}
added across R
F
to compensate for this noise gain
peaking. To achieve a flat transimpedance frequency
Where:
response, the pole in the feedback network should be
4.5kΩ ≤ R
OUT1
≤ 22.1kΩ
set to:
R
F
= 10kΩ
Using the previous values, the variation of the parallel
(2)
combination of R
F
and R
OUT1
can be constrained to:
4.19kΩ ≤ (R
F
R
OUT1
) ≤ 6.88kΩ. In order to optimize
which gives a closed-loop transimpedance
the bias current cancellation, we select R
S
to be the
bandwidth, f
–3dB
, of approximately:
average of those limiting numbers, or R
S
= (6.88kΩ +
4.19kΩ)/2 = 5.56kΩ.
(3)
Looking at the variation for each code, the total error
(when including all codes) is ~3.9LSB for the
Using the DAC7822 internal output capacitance of
OPA890.
25pF gives a feedback capacitance (C
F
) of 2.5pF and
an 8.8MHz bandwidth.
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