THS3215 Datasheet, PDF (38/71 Pages) Texas Instruments – THS3215 650-MHz, Differential to Single-Ended DAC Output Amplifier

THS3215
SBOS780A – MARCH 2016 – REVISED APRIL 2016
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Feature Description (continued)
At very low gains (< 1.5 V/V), parasitic effects at the inverting input due to stray inductance and capacitance
render a flat frequency response impossible. Looking then at gains from 1.5 V/V and up, a table of nominally
recommended R_F and R_G values is shown in Table 1. Do not operate the OPS in noninverting gains of less than
2.5 V/V for large output signals because the limited slew-rate of the CFA input buffer causes signal degradation.
Table 1 accounts for the nominal gain losses described previously, and uses standardized resistor values to
minimize the nominal gain-error to target gain. The calculation also restricts the solution to a minimum R_G = 20
Ω. The gain calculations include the nominal buffer gain loss, the loop-gain effect, and the nominal internal
feedback resistor = 18.5 kΩ.
TARGET GAIN
(V/V)
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10
Table 1. Optimized R_F Values for Different OPS Noninverting Signal Gains
MEASURED SSBW
(MHz)
BEST R_F
(Ω)
BEST R_G
(Ω)
CALCULATED GAIN
(V/V)
(dB)
890
324
634
1.498
3.513
—
287
280
2.004
6.039
700
249
162
2.510
7.995
—
205
102
2.980
9.485
—
169
66.5
3.510
10.905
—
150
49.9
3.972
11.980
—
158
44.2
4.533
13.128
390
158
39.2
4.984
13.952
—
165
36.5
5.467
14.755
—
169
33.2
6.029
15.605
—
169
30.1
6.547
16.321
—
174
28.7
6.989
16.888
—
174
26.7
7.437
17.429
—
178
24.9
8.060
18.127
—
178
23.2
8.578
18.668
—
178
22.1
8.955
19.041
—
182
21
9.558
19.608
93
187
20.5
10.006
20.005
GAIN ERROR
(%)
-0.1
0.22
0.41
-0.66
0.27
-0.7
0.73
-0.32
-0.6
0.49
0.73
-0.16
-0.83
0.75
0.92
-0.5
0.61
0.06
The measured bandwidths in Table 1 come from Figure 25 using the resistor values in the table and a 100-Ω
load. Plotting the R_F value versus gain gives the curve of Figure 83. The curve shows some ripple due to the
standard value resistors used to minimize the target dc gain error.
350
300
250
200
150
100
50
0
1 2 3 4 5 6 7 8 9 10
Target OPS Gain (V/V)
D503
Figure 83. Suggested External R_F Value vs Noninverting Gain for the OPS
38
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