AD7980 datasheet(13/24 Pages) AD

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Data Sheet

AD7625

Rev. A | Page 13 of 24

THEORY OF OPERATION

SW+

COMP

SW–

IN+

REF

(4.096V)

GND

LSB

MSB

32,768C 16,384C

CONTROL

LOGIC

CNV+, CNV–

IN–

32,768C 16,384C

LSB

MSB

GND

D+, D

–

CLK+, CLK–

DCO+, DCO

–

LVDS INTERFACE

DATA TRANSFER

CONVERSION

CONTROL

OUTPUT CODE

SWITCHES

CONTROL

Figure 18. ADC Simplified Schematic

CIRCUIT INFORMATION

The AD7625 is a 6 MSPS, high precision, power efficient, 16-bit

ADC that uses SAR based architecture to provide performance

of 93 dB SNR, ±0.45 LSB INL, and ±0.3 LSB DNL.

The AD7625 is capable of converting 6,000,000 samples per

second (6 MSPS). The device typically consumes 135 mW. The

AD7625 offers the added functionality of a high performance

on-chip reference and on-chip reference buffer.

The AD7625 is specified for use with 5 V and 2.5 V supplies

(VDD1, VDD2). The interface from the digital host to the

AD7625 uses 2.5 V logic only. The AD7625 uses an LVDS

interface to transfer data conversions. The CNV+ and CNV−

inputs to the part activate the conversion of the analog input.

The CNV+ and CNV− pins can be applied using a CMOS or

LVDS source.

The AD7625 is housed in a space-saving, 32-lead, 5 mm ×

5 mm LFCSP.

CONVERTER INFORMATION

The AD7625 is a 6 MSPS ADC that uses SAR based architecture

incorporating a charge redistribution DAC. Figure 18 shows a

simplified schematic of the ADC. The capacitive DAC consists

of two identical arrays of 16 binary weighted capacitors that are

connected to the two comparator inputs.

During the acquisition phase, the terminals of the array tied

to the input of the comparator are connected to GND via SW+

and SW−. All independent switches are connected to the analog

inputs. In this way, the capacitor arrays are used as sampling

capacitors and acquire the analog signal on the IN+ and IN−

inputs. A conversion phase is initiated when the acquisition

phase is complete and the CNV± input goes logic high. Note

that the AD7625 can receive a CMOS (CNV+) or LVDS format

(CNV±) signal.

When the conversion phase begins, SW+ and SW− are opened

first. The two capacitor arrays are then disconnected from the

inputs and connected to the GND input. Therefore, the differential

voltage between the inputs (IN+ and IN−) captured at the end

of the acquisition phase is applied to the comparator inputs,

causing the comparator to become unbalanced. By switching

each element of the capacitor array between GND and 4.096 V

(the reference voltage), the comparator input varies by binary

weighted voltage steps (VREF/2, VREF/4 … VREF/65,536). The

control logic toggles these switches, MSB first, to bring the

comparator back into a balanced condition. At the completion

of this process, the control logic generates the ADC output code.

The AD7625 digital interface uses low voltage differential

signaling (LVDS) to enable high data transfer rates.

The AD7625 conversion result is available for reading after tMSB

(time from the conversion start until MSB is available) has

elapsed. The user must apply a burst LVDS CLK± signal to the

AD7625 to transfer data to the digital host.

The CLK± signal outputs the ADC conversion result onto the

data output D±. The bursting of the CLK± signal is illustrated

in Figure 29 and Figure 30 and is characterized as follows: The

differential voltage on CLK± should be held to create logic low

in the time between tCLKL and tMSB.

The AD7625 has two data read modes. For more information

about the echoed-clock and self-clocked interface modes, see

the Digital Interface section.

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