AD7655AST datasheet(14/28 Pages) AD

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AD7655

Rev. B | Page 14 of 28

APPLICATION INFORMATION

CIRCUIT INFORMATION

The AD7655 is a very fast, low power, single-supply, precise

simultaneous sampling 16-bit ADC.

The AD7655 provides the user with two on-chip, track-and-

hold, successive approximation ADCs that do not exhibit any

pipeline or latency, making it ideal for multiple multiplexed

channel applications. The AD7655 can also be used as a

4-channel ADC with two pairs simultaneously sampled.

The AD7655 can be operated from a single 5 V supply and be

interfaced to either 5 V or 3 V digital logic. It is housed in a

48-lead LQFP or a tiny, 48-lead LFCSP that combines space

savings and allows flexible configurations as either a serial or

parallel interface. The AD7655 is pin-to-pin compatible with

PulSAR ADCs.

MODES OF OPERATION

The AD7655 features two modes of operation, normal mode

and impulse mode. Each of these modes is suitable for specific

applications.

Normal mode is the fastest mode (1 MSPS). Except when it is

powered down (PD = HIGH), the power dissipation is almost

independent of the sampling rate.

Impulse mode, the lowest power dissipation mode, allows

power saving between conversions. The maximum throughput

in this mode is 888 kSPS. When operating at 20 kSPS, for

example, it typically consumes only 2.6 mW. This feature makes

the AD7655 ideal for battery-powered applications.

TRANSFER FUNCTIONS

The AD7655 data format is straight binary. The ideal transfer

characteristic for the AD7655 is shown in Figure 16 and Table 7.

The LSB size is 2 × VREF/65536, which is about 76.3 μV.

000...000

000...001

000...010

111...101

111...110

111...111

ANALOG INPUT

+FS – 1.5 LSB

+FS – 1 LSB

–FS + 1 LSB

–FS

–FS + 0.5 LSB

Figure 16. ADC Ideal Transfer Function

Table 7. Output Codes and Ideal Input Voltages

Description

Analog Input

VREF = 2.5 V

Digital Output Code

FSR − 1 LSB

4.999924 V

0xFFFF1

FSR − 2 LSB

4.999847 V

0xFFFE

Midscale + 1 LSB

2.500076 V

0x8001

Midscale

2.5 V

0x8000

Midscale − 1 LSB

2.499924 V

0x7FFF

−FSR + 1 LSB

−76.29 μV

0x0001

−FSR

0 V

0x00002

1 This is also the code for overrange analog input:

– V

above 2 × (V

– V

)).

INx

INxN

REF

REFGND

2 This is also the code for underrange analog input (VINx below VINxN).

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