REV. 0

AD6140

–6–

POWER SUPPLY VOLTAGE – Volts

5.2

4.8

4.4

2.5

5.0

4.6

2.6

2.7

2.8

2.9

Figure 7. Supply Current vs. Power Supply Voltage

MIXER POST

AMPLIFIER

MIXER

MODULATOR

AGC

DETECTOR

CIRCUIT

ECL-TO-CMOS

LEVEL-SHIFTER

LNA BIAS

AMPLIFIER

PREAMPLIFIER

IF_INPUT

LNA_SENSE

LNA_FORCE

_DATA_OUT

_CLOCK_OUT

BUFFER_VDD

CLK_IN+

CLK_IN–

BUFFER_GND

AVDD AGND DGND DVDD POWER_DOWN

AGC_CAPACITOR

0.1 F

BIAS_RESISTOR

39k

BIAS SYSTEM

AGC_TC_SELECT

VOLTAGE_REFERENCE_IN

AD6140

LO_IN+

LO_IN–

Figure 8. Functional Block Diagram

MODULATION

A

Σ∆ modulator uses feedback around a low noise quantizer

(1 bit in this case) in order to “shape” the spectrum of quantiza-

tion noise. Using this technique, we can shape noise away from

an arbitrary passband, within which we can place a modulated

signal. A

Σ∆ modulator reproduces the input, but adds quanti-

zation noise, which can be digitally removed with a filter, known

as a decimation filter. Applying this technique to bandpass

signals results in an analog-to-digital converter suitable for

converting the IF signals in a digital radio.

The output of the AD6140’s

Σ∆ modulator is shown in Figure

9. As can be seen, the noise is shaped away from a narrow band-

width, within which we place a signal (a sine wave in this case)

resulting in a narrowband, high dynamic range digital represen-

tation of the analog input.

FREQUENCY – kHz

0

–100

–150

0

–50

500

1000

1500

2000

2500

3000

3500

Figure 9. Output Spectrum of AD6140

PRODUCT OVERVIEW

The AD6140 is a bandpass

Σ∆ analog-to-digital converter IF

IC for dual conversion receivers requiring a high dynamic range

and multiple filter bandwidths. It consists of a variable gain, low

noise preamplifier, mixer, automatic gain control (AGC) detec-

tor, bandpass

Σ∆ modulator, an ECL to CMOS level translator

and an auxiliary amplifier for use in biasing a discrete LNA.

The low noise preamplifier accepts a first IF input at 49.6 MHz

from 16.4

µV p-p to 63.2 mV p-p. It provides a variable gain

from 12 dB to 25 dB.

The mixer accepts an LO frequency of 49.792 MHz or

49.408 MHz, resulting in an IF frequency of 192 kHz. The LO

level should be 200 mV p-p differential. It is ac-coupled to the

AD6140. The mixer operates in the linear region, hence the

gain of the mixer is a function of the LO level. As a result, spe-

cial care must be taken to ensure that the LO level is 200 mV p-p,