AD9864-EBZ datasheet(29/48 Pages) AD

29 / 48 page

AD9864

Data Sheet

Rev. A | Page 28 of 47

IF LNA/MIXER

The AD9864 contains a single-ended LNA followed by a Gilbert

type active mixer, shown in Figure 44 with the required external

components. The LNA uses negative shunt feedback to set its

input impedance at the IFIN pin, thus making it dependent on

the input frequency. It can be modeled as approximately

370 Ω||1.4 pF (±20%) below 100 MHz. Figure 45 and Figure 46

show the equivalent input impedance vs. frequency characteristics

of the AD9864. The increase in shunt resistance vs. frequency

can be attributed to the reduction in bandwidth, thus the amount

of negative feedback of the LNA. Note that the input signal into

IFIN must be ac-coupled via a 10 nF capacitor because the LNA

input is self biasing.

IFIN

RBIAS

VDDI

MXOP

LO INPUT =

0.3V p-p TO

1.0V p-p

DC SERVO

LOOP

MULTI-TANH

V–I STAGE

CXIF

MXON

CXVM

CXVL

2.7V TO 3.6V

RGAIN

50Ω

Figure 44. Simplified Schematic of AD9864 LNA/Mixer

FREQUENCY (MHz)

500

100

400

600

200

550

450

150

350

300

350

300

250

Figure 45. Shunt Input Resistance vs. Frequency of AD9864 IF1 Input

FREQUENCY (MHz)

1.5

100

0.5

2.5

200

2.0

1.0

150

350

300

250

Figure 46. Shunt Capacitance vs. Frequency of AD9864 IF1 Input

The differential LO port of the mixer is driven by the LO buffer

stage shown in Figure 44, which can be driven single-ended or

differential. Because it is self biasing, the LO signal level can be

ac-coupled and range from 0.3 V p-p to 1.0 V p-p with negligible

effect on performance. The open-collector outputs of the mixer,

MXOP and MXON, drive an external resonant tank consisting of

a differential LC network tuned to the IF of the band-pass Σ-∆

ADC, that is, fIF2_ADC = fCLK/8. The two inductors provide a dc

bias path for the mixer core via a series resistor of 50 Ω, which

is included to dampen the common-mode response. The output

of the mixer must be ac-coupled to the input of the band-pass

Σ-∆ ADC, IF2P, and IF2N via two 100 pF capacitors to ensure

proper tuning of the LC center frequency.

The external differential LC tank forms the resonant element for

the first resonator of the band-pass Σ-∆ modulator, and so must

be tuned to the fCLK/8center frequency of the modulator. The

inductors must be chosen such that their impedance at fCLK/8is

approximately 140, that is, L = 180/fCLK. An accuracy of 20% is

considered to be adequate. For example, at fCLK = 18 MHz, L =

10 µH is a good choice. Once the inductors have been selected,

the required tank capacitance may be calculated using the

relation

]

)

(

fCLK

For example, at fCLK = 18 MHz and L = 10 µH, a capacitance of

250 pF is needed. However, to accommodate an inductor tolerance

of ±10%, the tank capacitance must be adjustable from 227 pF

to 278 pF. Selecting an external capacitor of 180 pF ensures that

even with a 10% tolerance and stray capacitances as high as 30 pF,

the total capacitance is less than the minimum value needed by

the tank. Extra capacitance is supplied by the AD9864 on-chip

programmable capacitor array. Because the programming range

of the capacitor array is at least 160 pF, the AD9864 has plenty of

range to make up for the tolerances of low cost external

components. Note that if fCLK is increased by a factor of 1.44 MHz

to 26 MHz so that fCLK/8becomes 3.25 MHz, reducing L and C

by approximately the same factor (L = 6.9 µH and C = 120 pF)

satisfies the requirements stated previously.

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