ADS5547 datasheet(33/54 Pages) TI

33 / 54 page

www.ti.com

Using RF transformers

Using Differential Amplifier Drive Circuits

FIL

0.1 F

10 F

||R

INP

INM

ADS5517

THS4509

VCM

500 W

200 W

5 W

500 W

0.1 F

–V

ADS5517

SLWS203 – DECEMBER 2007

For optimum performance, the analog inputs have to be driven differentially. This improves the common-mode

noise immunity and even order harmonic rejection. The single-ended signal is fed to the primary winding of the

RF transformer. The transformer is terminated on the secondary side. Putting the termination on the secondary

side helps to shield the kickbacks caused by the sampling circuit from the RF transformer’s leakage inductances.

The termination is accomplished by two resistors connected in series, with the center point connected to the 1.5

V common-mode (VCM pin 13).

At higher input frequencies, 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 (Figure 34) may be required between the two transformers to improve the balance

between the P and M sides. The center point of this termination must be connected to ground. (Note that the

drive circuit has to be tuned to account for this additional termination, to get the desired S11 and impedance

match).

Figure 37 shows a drive circuit using a differential amplifier (TI's THS4509) to convert a single-ended input to

differential output that can be interface to the ADC analog input pins. In addition to the single-ended to differential

conversion, the amplifier also provides gain (10 dB in Figure 37). RFIL helps to isolate the amplifier outputs from

the switching input of the ADC. Together with CFIL, it forms a low-pass filter that band-limits the noise (and signal)

at the ADC input. As the amplifier output is ac-coupled, the common-mode voltage of the ADC input pins is set

using two 200

Ω resistors connected to VCM.

The amplifier output can also be dc-coupled. Using the output common-mode control of the THS4509, the ADC

input pins can be biased to 1.5 V. In this case, use +4 V and -1 V supplies for the THS4509 so that its output

common-mode voltage (1.5 V) is at mid-supply.

Figure 37. Drive Circuit Using the THS4509

See the EVM User Guide (SLWU028) for more information.

Submit Documentation Feedback

Product Folder Link(s): ADS5517

ADS5547 Datasheet(PDF) 33 Page - Texas Instruments

ADS5547 Datasheet(HTML) 33 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

Datasheet Download

Link URL