ADS54J60 Datasheet, PDF (66/76 Pages) Texas Instruments – Dual-Channel, 16-Bit, 1.0-GSPS Analog-to-Digital Converter

ADS54J60
SBAS706B – APRIL 2015 – REVISED AUGUST 2015
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Typical Application (continued)
8.2.1 Design Requirements
8.2.1.1 Transformer-Coupled Circuits
Typical applications involving transformer-coupled circuits are discussed in this section. Transformers (such as
ADT1-1WT or WBC1-1) can be used up to 300 MHz to achieve good phase and amplitude balances at the ADC
inputs. When designing dc driving circuits, the ADC input impedance must be considered. Figure 126 and
Figure 127 show the impedance (Z_IN = R_IN || C_IN) across the ADC input pins.
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0 100 200 300 400 500 600 700 800 900 1000
Frequency (MHz)
D103
5
4.75
4.5
4.25
4
3.75
3.5
3.25
3
2.75
2.5
2.25
0
100 200 300 400 500 600 700 800 900 1000
Frequency (MHz)
D102
Figure 126. R_IN vs Input Frequency
Figure 127. C_IN vs Input Frequency
By using the simple drive circuit of Figure 128, uniform performance can be obtained over a wide frequency
range. The buffers present at the analog inputs of the device help isolate the external drive source from the
switching currents of the sampling circuit.
0.1 F
T1
T2
0.1 F
5
CHx_INP
25
0.1 F
R_IN
C_IN
0.1 F
25
5
CHx_INM
1:1
1:1
Figure 128. Input Drive Circuit
Device
8.2.2 Detailed Design Procedure
For optimum performance, the analog inputs must be driven differentially. This architecture improves common-
mode noise immunity and even-order harmonic rejection. A small resistor (5 Ω to 10 Ω) in series with each input
pin is recommended to damp out ringing caused by package parasitics, as shown in Figure 128.
66
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