AD6640STSLASHPCB datasheet(12/25 Pages) AD

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AD6640

–11–

REV. A

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 V and should swing

±0.5 V around this

reference (see Figure 2). Since AIN and

AIN are 180 degrees out

of phase, the differential analog input signal is 2 V p-p.

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

A valid ENCODE clock must be present on the AD6640 before

the application of AVCC (5 V). Best performance is obtained by

driving the ENCODE pins differentially. 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

(see the first equation under the Noise Floor and SNR section).

0.01 F

TTL OR CMOS

SOURCE

ENCODE

AD6640

Figure 7. Single-Ended TTL /CMOS ENCODE

The AD6640 ENCODE inputs are connected to a differential

input stage (see Figure 3). With no input signal connected to

either ENCODE pin, the voltage dividers bias the inputs to

1.6 V. For TTL or CMOS usage, the ENCODE source should

be connected to ENCODE, Pin 3.

ENCODE should be decoupled

using a low inductance or microwave 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

and R2 = 8 k

Ω).

V l

5R2Rx

R1R2

+ R1Rx + R2Rx

to lower logic threshold.

0.01 F

ENCODE

SOURCE

ENCODE

AD6640

Figure 8. Lower Logic Threshold for ENCODE

RRx

l =

to raise logic threshold.

0.01 F

ENCODE

SOURCE

ENCODE

AD6640

AVCC

Figure 9. Raise Logic Threshold for ENCODE

While the single-ended ENCODE will work well for many appli-

cations, 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 recom-

mended that differential TTL logic be used 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

Ω, to the primary

side of an RF transformer (these transformers are inexpensive

and readily available; part number in Figure 10 is from Mini-

Circuits). The secondary side is connected to the ENCODE

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 10. TTL Source–Differential ENCODE

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

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

to match source and load impedances. The input impedance of

the AD6640 ENCODE is approximately 11 k

Ω differentially.

Therefore the “R,” shown in the Figure 11, may be any value

that is convenient for available drive power.

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