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APPLICATION INFORMATION
Stability Circuit
GND
I 1
OUT
R
FB
V
REF
V
REF
V
DD
V
DD
V
OUT
I 2
OUT
C
1
U1
U2
Amplifier Selection
DAC7821
SBAS365B – OCTOBER 2005 – REVISED JULY 2007
For a current-to-voltage design (see Figure 27 ), the DAC7821 current output (I
OUT
) and the connection with the
inverting node of the op amp should be as short as possible and according to correct printed circuit board (PCB)
layout design practices. For each code change, there is a step function. If the gain bandwidth product (GBP) of
the op amp is limited and parasitic capacitance is excessive at the inverting node, then gain peaking is possible.
Therefore, for circuit stability, compensation capacitor C
1
(1pF to 5pF typ) can be added to the design, as shown
in Figure 27 .
Figure 27. Gain Peaking Prevention Circuit with Compensation Capacitor
There are many choices and many differences in selecting the proper operational amplifier for a multiplying DAC
(MDAC). Making the analog signal out of the MDAC is one critical aspect. However, there are also other issues
to take into account such as amplifier noise, input bias current, and offset voltage, as well as MDAC resolution
and glitch energy. Table 1 and Table 2 suggest some suitable operational amplifiers for low power, fast settling,
and high-speed applications. A greater selection of operational amplifiers can be found at www.ti.com/amplifer .
Table 1. Suitable Precision Operational Amplifiers from Texas Instruments
I
Q
TOTAL TOTAL PER SLEW OFFSET
SUPPLY SUPPLY CHANNEL GBW RATE DRIFT I
IB
CMRR
VOLTAGE VOLTAGE (max) (typ) (typ) (typ) (max) (min) PACKAGE/
PRODUCT (V) (min) (V) (max) (mA) (MHz) (V/ μ s) ( μ V/ ° C) (pA) (dB) LEAD DESCRIPTION
Low Power
SOT5-23,
12V, CMOS, Rail-to-Rail I/O,
OPA703 4 12 0.2 1 0.6 4 10 70 PDIP-8,
Operational Amplifier
SOIC-8
0.05 μ V/ ° C (max),
SOT5-23, Single-Supply CMOS
OPA735 2.7 12 0.75 1.6 1.5 0.01 200 115
SOIC-8 Zero-Drift Series Operational
Amplifier
Low Power, Single-Supply,
SOT5-23,
Rail-To-Rail Operational
OPA344 2.7 5.5 0.25 1 1 2.5 10 80 PDIP-8,
Amplifiers MicroAmplifier
SOIC-8
Series
SC5-70, 1MHz, 45 μ A, Rail-to-Rail I/O,
OPA348 2.1 5.5 0.065 1 0.5 2 10 70 SOT5-23, Single
SOIC-8 Op Amp
PDIP-8,
High Precision Operational
OPA277 4 36 0.825 1 0.8 0.1 1000 130 SOIC-8,
Amplifiers
SON-8
Fast Settling
High-Speed, Single-Supply,
MSOP-8,
Rail-to-Rail Operational
OPA350 2.7 5.5 7.5 38 22 4 10 76 PDIP-8,
Amplifiers MicroAmplifier
SOIC-8
Series
e-trim 20MHz, High
MSOP-8,
OPA727 4 12 6.5 20 30 0.6 500 86 Precision CMOS Operational
SON-8
Amplifier
PDIP-8, High Precision, Low Noise
OPA227 5 36 3.8 8 2.3 0.1 10000 120
SOIC-8 Operational Amplifiers
12
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