LTC6948 datasheet(23/32 Pages) AD

23 / 32 page

LTC5594

Rev A

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APPLICATIONS INFORMATION

Table 5. Component Values for Anti-Alias Lowpass Filter

1dB BW

(MHz)

L5 – L8

(nH)

C17, C18

(pF)

C20, C21

(pF)

C23, C24

(pF)

560

180

240

100

120

300

3.9

8.2

6.8

500

1.8

6.8

3.3

1000

1.0

3.3

1.8

Tables 6 and 7 show the differential and common mode

S-parameters for the amplifier by itself with 50Ω termina-

tions on all ports. In addition, common mode terminations

were used on the input and output ports having a value of

2pF in series with 50Ω.

The common mode feedback amplifier holds the common

mode output voltage within about 20mV of the VCM pin

voltage. The VCM pin interface is shown in Figure 13. The

VCM pin should be driven by a voltage source with an

output impedance lower than 1kΩ. When the VCM pin is

unbiased, the output common mode voltage will be held

at a nominal 0.9V given by the internal voltage divider

formed by the 40k and 8k resistors. Connecting the VCM

pin to an ADC common mode reference pin allows the

output common mode voltage of the IF amplifier to track

the ADC common mode.

Table 6. IF Amplifier S-Parameters (Differential-Mode)

(MHz)

S11

S21

S12

S22

MAG

ANG

MAG

ANG

MAG

ANG

MAG

ANG

0.001 0.204 –179.9 2.129 180.0 1.8e-4 164.8 0.014 178.5

100 0.203 176.0 2.154 171.9 5.4e-4 118.0 0.026 –120.9

200 0.205 172.2 2.170 163.7 1.0e-4 102.8 0.050 –112.0

300 0.207 168.5 2.197 155.6 1.7e-4 92.8 0.079 –113.5

400 0.210 164.8 2.239 147.3 2.8e-4 93.7 0.111 –118.3

500 0.215 160.9 2.292 138.8 3.2e-4 95.4 0.147 –125.0

600 0.221 157.0 2.363 130.1 4.0e-4 92.0 0.186 –132.1

700 0.227 153.0 2.445 121.2 5.0e-4 92.1 0.230 –140.0

800 0.235 149.0 2.535 112.0 5.5e-4 86.2 0.279 –148.1

900 0.242 144.6 2.642 102.0 6.9e-4 93.2 0.334 –157.0

1000 0.251 140.6 2.770 92.3 7.9e-4 92.7 0.396 –166.2

1500 0.303 117.6 3.420 32.3

0.003

92.6 0.738 134.4

2000 0.365 90.2 3.318 –45.5 0.005

33.2 0.828 70.0

2500 0.385 56.1 2.232 –105.2 0.005

–3.1 0.666 13.1

3000 0.365 16.6 2.620 –160.2 0.005 –34.2 0.488 –38.4

3500 0.319 –28.2 1.021 157.4 0.005 –61.9 0.418 –94.7

4000 0.307 –83.4 0.742 113.3 0.005 –79.5 0.409 –150.6

Table 7. IF Amplifier S-Parameters (Common Mode)

(MHz)

S11

S21

S12

S22

MAG

ANG

MAG

ANG

MAG

ANG

MAG

ANG

0.001 0.184 –138.7 9.2e-4 –112.8 0.037 –65.3 0.985 179.8

100 0.186 172.5 0.085 –118.9 0.013 –68.6 0.152 126.7

200 0.188 166.6 0.173 –134.7 0.007 –91.8 0.125 116.7

300 0.191 160.2 0.237 –150.0 0.004 –113.1 0.097 97.3

400 0.196 154.4 0.291 –163.8 0.002 –145.4 0.067 75.2

500 0.202 148.4 0.340 –176.8 0.002 170.2 0.037 43.6

600 0.210 142.8 0.387 170.9 0.002 137.0 0.023 –38.0

700 0.219 137.2 0.436 159.1 0.003 118.1 0.051 –97.8

800 0.230 132.0 0.488 147.1 0.003 107.8 0.094 –121.5

900 0.243 126.5 0.550 134.9 0.004 106.6 0.148 –137.0

1000 0.252 120.9 0.612 122.2 0.006 104.8 0.211 –151.3

1500 0.325 96.7

0.981

43.4 0.020 80.4 0.749 136.1

2000 0.438 72.1

0.776 –46.1 0.036 18.6 1.005 55.9

2500 0.549 40.1

0.496 –97.1 0.041 –21.9 0.873

2.9

3000 0.601

6.9

0.397 –143.2 0.042 –52.2 0.764 –37.3

3500 0.618 –27.5 0.281 –175.7 0.044 –80.3 0.668 –72.7

4000 0.595 –60.3 0.254 147.3 0.046 –101.2 0.620 –107.0

Figure 13. Simplified Schematic of the VCM Input Pin

250

40k

6.5pF

20k

LTC5594

GND

VCM

5594 F13

VCC

–

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