THS3217 Datasheet, PDF (38/73 Pages) Texas Instruments – THS3217 DC to 800-MHz, Differential-to-Single-Ended, DAC Output Amplifier

THS3217
SBOS766B – FEBRUARY 2016 – REVISED FEBRUARY 2016
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Feature Description (continued)
At very low gains (< 1.5 V/V) parasitic LC effects at the inverting input 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 2. 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 2 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 2. 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)
1400
294
562
1.505
3.551
—
274
267
1.998
6.013
950
249
162
2.500
7.960
—
232
113
3.008
9.566
—
205
80.6
3.493
10.863
—
182
59
4.028
12.103
—
165
46.4
4.495
13.055
652
140
34.8
4.960
13.910
—
121
26.7
5.467
14.754
—
113
22.1
6.043
15.624
—
115
20.5
6.532
16.301
—
121
20
6.965
16.859
—
133
20
7.553
17.563
—
143
20
8.043
18.108
—
154
20
8.580
18.670
—
162
20
8.971
19.057
—
174
20
9.557
19.606
315
187
20.5
9.966
19.970
GAIN ERROR
(%)
0.3
–0.1
0
0.3
–0.2
0.7
–0.1
–0.8
–0.6
0.7
0.5
–0.5
0.7
0.5
0.9
–0.3
0.6
–0.3
The measured bandwidths in Table 2 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
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