Datasheet

REV. 0–16–
AD8369
For example, suppose the input signal in Figure 4 is a 140 MHz
sinusoid from a ground-referenced 50 W source. The 0.1 mF
coupling capacitors present a very low reactance at this frequency
(11 mW) so that essentially all of the ac voltage is delivered to the
differential inputs of the AD8369. It will be apparent that, in
addition to the use of adequate coupling capacitance, the external
capacitor used to extend the low frequency range of the offset
control loop, C
FILT
, must also be large enough to prevent the
offset control loop from attempting to track the ac signal fluctuations.
Interfacing to an ADC
The AD8369 can be used to effectively increase the dynamic
range of an ADC in a direct IF sampling receiver application.
Figure 5 provides an example of an interface to an ADC designed
for an IF of 70 MHz. It comprises a low-pass filter that attenuates
harmonics while providing an impedance transformation from
200 W to 1 kW. This impedance transformation allows the AD8369
to operate much below its peak output swing in the pass band,
which significantly reduces distortion.
0.1␮F
CONTROL INTERFACE
910111213141516
PWUP VPOS SENB FILT CMDC OPHI
INHI
COMM
INLO DENBCOMM BIT0 BIT1 BIT2 BIT3
AD8369
OPLO
0.1␮F
V
S
87654321
0.1␮F
0.1␮F
0.1␮F
6.8pF 15pF
1k⍀
270nH
270nH
ADC
Figure 5. AD8369 to ADC Interface
ADC OUTPUT FREQUENCY – MHz
0
–10
0
P
OUT
– dBFS
–20
–30
–40
–50
–60
–70
–80
–90
–100
51015202530
70MHz – 1dBFS
HD2 = –83dBc
HD3 = –80dBc
SNR = 51dB
Figure 6. Single-Tone 70 MHz, –1 dBFS
ADC OUTPUT FREQUENCY – MHz
0
–10
0
P
OUT
– dBFS
–20
–30
–40
–50
–60
–70
–80
–90
–100
51015202530
–72.5dBFS
–7dBFS
Figure 7. Two-Tone, 70 MHz, 70.3 MHz, –7 dBFS
A high performance 14-bit ADC, the AD6645, is used for illus-
trative purposes and is sampling at 64 MSPs with a full-scale
input of 2.2 V p-p. Typically, an SNR of 51 dB and an SFDR of
almost –90 dBFS are realized by this configuration. Figure 6 shows
an FFT of the AD8369 delivering a single tone at –1 dBFS (that
is, 2 V p-p) at the input of the ADC with an HD2 of –83 dBc and
HD3 of –80 dBc. Figure 7 shows that the two-tone, third-order
intermodulation distortion level is –65.5 dBc.
PCB Layout Considerations
Each input and output pin of the AD8369 presents 100 W
rela
tive to their respective ac grounds. To ensure that signal
integrity is not seriously impaired by the printed circuit board, the
relevant connection traces should provide a characteristic
impedance of 100 W to the ground plane. This can be achieved
through proper layout. Figure 8 shows the cross section of a PC
board and Table II shows the dimensions that will provide a
100 W line impedance.
Table II. Dimensions Required for 100 W Characteristic
Impedance Microstrip Line in FR-4

r
(FR-4) 4.6
W22 mils
H53 mils
T 2.1 mils