ADS7816UC Datasheet, PDF (42/59 Pages) Texas Instruments – Quad-Channel, 250-MSPS Receiver and Feedback ADC

ADS58H40
SBAS589B – AUGUST 2012 – REVISED NOVEMBER 2012
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Driving Circuit
Two example driving circuits with a 50-Ω source impedance are shown in Figure 46 and Figure 47. The driving
circuit in Figure 46 is optimized for input frequencies in the second Nyquist zone (centered at 185 MHz), whereas
the circuit in Figure 47 is optimized for input frequencies in third Nyquist zone (centered at 310 MHz).
Note that both drive circuits are terminated by 50 Ω near the ADC side. This termination is accomplished with a
25-Ω resistor from each input to the 1.15-V common-mode (VCM) from the device. This architecture 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.
50 :
T1
1:1
T2
0.1 PF
10 :
INP
25 :
25 :
Band-Pass Filter
Centered at
f0 = 185 MHz
BW = 125 MHz
0.1 PF
82 nH
10 pF
RIN
CIN
25 :
25 :
INM
1:1
0.1 PF
10 :
VCM
Device
Figure 46. Driving Circuit for a 50-Ω Source Impedance and Input Frequencies
in the Second Nyquist Zone
50 :
T1
1:1
T2
0.1 PF
10 :
INP
25 :
25 :
Band-Pass Filter
Centered at
f0 = 310 MHz
BW = 125 MHz
0.1 PF
25 :
27 nH
25 :
10 pF
RIN
INM
1:1
0.1 PF
10 :
VCM
Figure 47. Driving Circuit for a 50-Ω Source Impedance and Input Frequencies
in the Third Nyquist Zone
CIN
Device
42
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