AD9864BCPZ datasheet(26/44 Pages) AD

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AD9864

FREQUENCY (MHz)

500

100

400

600

200

550

450

150

350

300

350

300

250

Table 13. SPI Registers Associated with CLK Synthesizer

Address

(Hex)

Bit

Breakdown

Width

Default

Value

Name

0x00

(7:0)

0xFF

STBY

0x01

(3:2)

CKOB

0x10

(5:0)

CKR (13:8)

0x11

(7:0)

0x38

CKR (7:0)

0x12

(4:0)

0x00

CKN (12:8)

0x13

(7:0)

0x3C

CKN (7:0)

0x14

(6)

(5)

(4:2)

(1:0)

CKF

CKINV

CKI

CKTM

0x15

(3:0)

0x0

CKFA (13:8)

0x16

(7:0)

0x04

CKFA (7:0)

Figure 44. The Shunt Input Resistance vs.

the Frequency of the AD9864’s IF1 Input

FREQUENCY (MHz)

1.5

100

0.5

2.5

200

2.0

1.0

150

350

300

250

IF LNA/MIXER

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

bert-type active mixer, shown in Figure 43 with the required

external components. The LNA uses negative shunt feedback to

set its input impedance at the IFIN pin, thus making it depend-

ent on the input frequency. It can be modeled as approximately

370 Ω||1.4 pF (±20%) below 100 MHz. Figure 44 and Figure 45

show the equivalent input impedance versus frequency charac-

teristics of the AD9864. The increase in shunt resistance versus

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 should be ac-coupled via a 10 nF capaci-

tor since the LNA input is self-biasing.

Figure 45. The Shunt Capacitance vs. the Frequency of the AD9864’s IF1 Input

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

Ω

The mixer’s differential LO port is driven by the LO buffer

stage shown in Figure 43, which can be driven single-ended or

differential. Since 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 negligi-

ble effect on performance. The mixer’s open-collector outputs,

MXOP and MXON, drive an external resonant tank consisting

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

Σ-∆ ADC, i.e., 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

mixer’s output 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/8 center frequency of the modulator. The

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

about 140, i.e., 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

Figure 43. Simplified Schematic of AD9864’s LNA/Mixer

Rev. 0 | Page 26 of 44

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