AD6640AST datasheet(11/24 Pages) AD

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AD6640

REV. 0

–11–

THEORY OF OPERATION

The AD6640 analog-to-digital converter (ADC) employs a two-

stage subrange architecture. This design approach ensures

12-bit accuracy, without the need for laser trim, at low power.

As shown in the functional block diagram, the AD6640 has

complementary analog input pins, AIN and

AIN. Each analog

input is centered at 2.4 volts and should swing

± 0.5 volts

around this reference (ref. Figure 2). Since AIN and

AIN are

180 degrees out of phase, the differential analog input signal is

2 volts peak-to-peak.

Both analog inputs are buffered prior to the first track-and-hold,

TH1. The high state of the ENCODE pulse places TH1 in

hold mode. The held value of TH1 is applied to the input of a

6-bit coarse ADC. The digital output of the coarse ADC drives

a 6-bit DAC; the DAC is 12 bits accurate. The output of the 6-

bit DAC is subtracted from the delayed analog signal at the

input of TH3 to generate a residue signal. TH2 is used as an

analog pipeline to null out the digital delay of the coarse ADC.

The 6-bit coarse ADC word and 7-bit residue word are added

together and corrected in the digital error correction logic to

generate the output word. The result is a 12-bit parallel digital

CMOS-compatible word, coded as twos complement.

APPLYING THE AD6640

Encoding the AD6640

Best performance is obtained by driving the encode pins dif-

ferentially. However, the AD6640 is also designed to interface

with TTL and CMOS logic families. The source used to drive

the ENCODE pin(s) must be clean and free from jitter. Sources

with excessive jitter will limit SNR (reference Equation 1 under

“Noise Floor and SNR”).

0.01 F

TTL OR CMOS

SOURCE

ENCODE

AD6640

Figure 25. Single-Ended TTL /CMOS Encode

The AD6640 encode inputs are connected to a differential input

stage (see Figure 3 under EQUIVALENT CIRCUITS). With

no input signal connected to either ENCODE pin, the voltage

dividers bias the inputs to 1.6 volts. For TTL or CMOS usage,

the encode source should be connected to ENCODE, Pin 3.

ENCODE should be decoupled using a low inductance or mi-

crowave chip capacitor to ground.

If a logic threshold other than the nominal 1.6 V is required, the

following equations show how to use an external resistor, Rx, to

raise or lower the trip point (see Figure 3; R1 = 17 k

Ω, R2 = 8 kΩ).

V l =

5R2Rx

R1R2

+ R1Rx + R2Rx

to lower logic threshold.

0.01 F

ENCODE

SOURCE

ENCODE

AD6640

+5V

Figure 26. Lower Logic Threshold for Encode

V l =

5R2

R1RX

+ R

to raise logic threshold.

0.01 F

ENCODE

SOURCE

ENCODE

AD6640

+5V

AVCC

Figure 27. Raise Logic Threshold for Encode

While the single-ended encode will work well for many applica-

tions, driving the encode differentially will provide increased

performance. Depending on circuit layout and system noise, a

1 dB to 3 dB improvement in SNR can be realized. It is not

recommended that differential TTL logic be used however,

because most TTL families that support complementary outputs

are not delay or slew rate matched. Instead, it is recommended

that the encode signal be ac-coupled into the ENCODE and

ENCODE pins.

The simplest option is shown below. The low jitter TTL signal

is coupled with a limiting resistor, typically 100 ohms, to the

primary side of an RF transformer (these transformers are inex-

pensive and readily available; part number in Figure 28 is from

Mini-Circuits). The secondary side is connected to the EN-

CODE and

ENCODE pins of the converter. Since both encode

inputs are self-biased, no additional components are required.

TTL

ENCODE

AD6640

100

T1–1T

0.1 F

Figure 28. TTL Source – Differential Encode

A clean sine wave may be substituted for a TTL clock. In this

case, the matching network is shown below. Select a transformer

ratio to match source and load impedances. The input impedance

of the AD6640 encode is approximately 11 k

Ω differentially.

Therefore “R,” shown in the Figure 29, may be any value that is

convenient for available drive power.

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