OP249_07 Datasheet, PDF (15/20 Pages) Analog Devices – Dual, Precision JFET High Speed Operational Amplifier

OP249
DAC OUTPUT AMPLIFIER
Unity-gain stability, a low offset voltage of 300 μV typical, and a
fast settling time of 870 ns to 0.01%, makes the OP249 an ideal
amplifier for fast DACs.
For CMOS DAC applications, the low offset voltage of the
OP249 results in excellent linearity performance. CMOS DACs,
such as the PM7545, typically have a code-dependent output
resistance variation between 11 kΩ and 33 kΩ. The change in
output resistance, in conjunction with the 11 kΩ feedback resistor,
results in a noise gain change, which causes variations in the
offset error, increasing linearity errors. The OP249 features
low offset voltage error, minimizing this effect and maintaining
12-bit linearity performance over the full-scale range of the
converter.
Because the DAC output capacitance appears at the inputs of
the op amp, it is essential that the amplifier be adequately
compensated. Compensation increases the phase margin and
ensures an optimal overall settling response. The required lead
compensation is achieved with Capacitor C in Figure 48.
V_DD
0.1µF
75Ω
REFERENCE
OR V_IN
18
20
V_DD
R_FB
OUT₁ 1
500Ω ^{19 VREF PM7545}
AGND 2
DB₁₁ TO DB₀ ^DGND
3
12
C
33pF
+15V
0.1µF
2
8
1/2
OP249
1
3
4 0.1µF
–15V
V_OUT
DATA INPUT
Figure 47. Fast Settling and Low Offset Error of the OP249 Enhances CMOS DAC Performance—Unipolar Operation
R4
20kΩ
1%
REFERENCE
OR V_IN
V_DD
0.1µF
75Ω
500Ω
18
20
V_DD
R_FB
OUT₁ 1
19 V_{REF PM7545}
AGND 2
DB₁₁ TO DB₀ ^DGND
3
12
DATA INPUT
C
33pF
+15V
0.1µF
2
8
1/2
OP249
1
3
R3
10kΩ
1%
R5
10kΩ
1%
5
1/2
OP249 ⁷
6 ₄ 0.1µF
–15V
V_OUT
Figure 48. Fast Settling and Low Offset Error of the OP249 Enhances CMOS DAC Performance—Bipolar Operation
Rev. F | Page 15 of 20