AD7665AST datasheet(12/24 Pages) AD

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REV. 0

AD7665

–12–

SWA

COMP

SWB

IND

REF

REFGND

LSB

MSB

32,768C

INGND

16,384C

CONTROL

LOGIC

SWITCHES

CONTROL

BUSY

OUTPUT

CODE

INC

INA

INB

CNVST

65,536C

Figure 3. ADC Simplified Schematic

CIRCUIT INFORMATION

The AD7665 is a fast, low-power, single-supply, precise 16-bit

analog-to-digital converter (ADC). The AD7665 features different

modes to optimize performances according to the applications.

In Warp mode, the AD7665 is capable of converting 570,000

samples per second (570 kSPS).

The AD7665 provides the user with an on-chip track/hold,

successive approximation ADC that does not exhibit any pipe-

line or latency, making it ideal for multiple multiplexed channel

applications.

It is specified to operate with both bipolar and unipolar input

ranges by changing the connection of its input resistive scaler.

The AD7665 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 package that combines space savings and flexible

configurations as either serial or parallel interface. The AD7665

is a pin-to-pin-compatible upgrade of the AD7663 and AD7664.

CONVERTER OPERATION

The AD7665 is a successive approximation analog-to-digital

converter based on a charge redistribution DAC. Figure 3 shows

the simplified schematic of the ADC. The input analog signal is,

first, scaled down and level-shifted by the internal input resistive

scaler which allows both unipolar ranges (0 V to 2.5 V, 0 V to

5 V, and 0 to 10 V) and bipolar ranges (

±2.5 V, ±5 V, and ±10 V).

The output voltage range of the resistive scaler is always 0 V to

2.5 V. The capacitive DAC consists of an array of 16 binary

weighted capacitors and an additional “LSB” capacitor. The

comparator’s negative input is connected to a “dummy” capaci-

tor of the same value as the capacitive DAC array.

During the acquisition phase, the common terminal of the array

tied to the comparator’s positive input is connected to AGND

via SWA. All independent switches are connected to the output

of the resistive scaler. Thus, the capacitor array is used as a

sampling capacitor and acquires the analog signal. Similarly, the

“dummy” capacitor acquires the analog signal on INGND input.

When the acquisition phase is complete, and the

CNVST input

goes or is low, a conversion phase is initiated. When the conversion

phase begins, SWA and SWB are opened first. The capacitor array

and the “dummy” capacitor are then disconnected from the inputs

and connected to the REFGND input. Therefore, the differen-

tial voltage between the output of the resistive scaler and INGND

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

REFGND or REF, the comparator input varies by binary

weighted voltage steps (VREF/2, VREF/4 ...VREF/65536). The

control logic toggles these switches, starting with the MSB first,

in order to bring the comparator back into a balanced condition.

After the completion of this process, the control logic generates

the ADC output code and brings BUSY output low.

Modes of Operation

The AD7665 features three modes of operations, Warp, Normal,

and Impulse. Each of these modes is more suitable for specific

applications.

The Warp mode allows the fastest conversion rate up to 570 kSPS.

However, in this mode, and this mode only, the full specified accu-

racy is guaranteed only when the time between conversion does

not exceed 1 ms. If the time between two consecutive conversions

is longer than 1 ms, for instance, after power-up, the first conver-

sion result should be ignored. This mode makes the AD7665

ideal for applications where both high accuracy and fast sample

rate are required.

The normal mode is the fastest mode (500 kSPS) without any

limitation about the time between conversions. This mode makes

the AD7665 ideal for asynchronous applications such as data

acquisition systems, where both high accuracy and fast sample

rate are required.

The impulse mode, the lowest power dissipation mode, allows

power saving between conversions. The maximum throughput

in this mode is 444 kSPS. When operating at 100 SPS, for ex-

ample, it typically consumes only 15

µW. This feature makes the

AD7665 ideal for battery-powered applications.

Transfer Functions

Using the OB/

2C digital input, the AD7665 offers two output

codings: straight binary and two’s complement. The ideal transfer

characteristic for the AD7665 is shown in Figure 4 and Table III.

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 4. ADC Ideal Transfer Function

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