ADS5560 datasheet(28/62 Pages) TI2

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S0329-01

1:1

0.1 F

INP

INM

VCM

ADS556x

50 W

22 pF

25 W

15 W

50 W

0.1 F

39 nH

0.1 F

f − Frequency − MHz

100

200

300

400

500

100

0.1

0.01

G045

f − Frequency − MHz

100

200

300

400

500

G046

ADS5560, ADS5562

SLWS207B – MAY 2008 – REVISED JANUARY 2016

www.ti.com

Feature Description (continued)

Figure 47. ADC Analog Input Capacitance (Cin) Across

Figure 46. ADC Analog Input Resistance (Rin) Across

Frequency

7.3.2.2 Example Driving Circuit

Figure 48 shows an example input configuration using RF transformers. In this example, an external R-C-R filter

using a 22-pF capacitor has been used. Together with the series inductor (39 nH), this combination forms a filter

and absorbs the sampling glitches. Because of the relatively large capacitor (22 pF) in the R-C-R and the 15-

resistors in series with each input pin, this drive circuit has low bandwidth and is suited for low input frequencies.

The drive circuit has been terminated by 50

Ω near the ADC side. The termination is accomplished by a 25-Ω

resistor from each input to the 1.5-V common-mode (VCM) from the device. This allows the analog inputs to be

biased around the required common-mode voltage.

The mismatch in the transformer parasitic capacitance (between the windings) results in degraded even-order

harmonic performance. Connecting two identical RF transformers back to back helps minimize this mismatch and

good performance is obtained for high frequency input signals. An additional termination resistor pair may be

required between the two transformers (enclosed by the dashed lines in Figure 48). The center point of this

termination is connected to ground to improve the balance between the P and M sides. The values of the

terminations between the transformers and on the secondary side must be chosen to get an effective 50

Ω (in

the case of 50-

Ω source impedance).

Figure 48. Drive Circuit Using RF Transformers

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