AD7680

–8–

REV. PrE

PRELIMINARY TECHNICAL DATA

ADC TRANSFER FUNCTION

The output coding of the AD7680 is straight binary. The

designed code transitions occur at successive integer LSB

values (i.e., 1 LSB, 2 LSBs, etc.). The LSB size is =

AD7680 is shown in Figure 5.

Figure 5. AD7680 Transfer Characteristic

000...000

0V

ANALOG INPUT

111...111

000...001

000...010

111...110

111...000

011...111

1LSB

+VDD-1LSB

1LSB = VDD/65536

and hold switch). This resistor is typically about 100

Ω.

The capacitor C2 is the ADC sampling capacitor and has

a capacitance of 30pF typically. For ac applications, re-

moving high frequency components from the analog input

signal is recommended by use of an RC low-pass filter on

the relevant analog input pin. In applications where har-

monic distortion and signal to noise ratio are critical the

analog input should be driven from a low impedance

source. Large source impedances will significantly affect

the ac performance of the ADC. This may necessitate the

use of an input buffer amplifier. The choice of the op amp

will be a function of the particular application. When no

amplifier is used to drive the analog input the source im-

pedance should be limited to low values. The maximum

source impedance will depend on the amount of total har-

monic distortion (THD) that can be tolerated. The THD

will increase as the source impedance increases and per-

formance will degrade (see TPC4).

TYPICAL CONNECTION DIAGRAM

Figure 6 shows a typical connection diagram for the

should be well decoupled. This provides an analog input

24-bit word, or alternatively all 16 bits of the conversion

result may be accessed using a minimum of 20 SCLKs.

This 20-/24-bit data stream consists of a four leading

zeros, followed by the 16 bits of conversion data followed

by four trailing zeros in the case of the 24 SCLK transfer.

For applications where power consumption is of concern,

the power-down mode should be used between conversions

or bursts of several conversions to improve power perfor-

mance. See Modes of Operation section of the datasheet.

In fact, because the supply current required by the

AD7680 is so low, a recision reference can be used as the

supply source to the AD7680. For example, a REF19x

voltage reference ( REF195 for 5 V or REF193 for 3 V

etc.) can be used to supply the required voltage to the

ADC (see Figure 6). This configuration is especially use-

ful if the power supply available is quite noisy or if the

system supply voltages are at some value other than the

required operating voltage of the AD7680 (e.g. 15V). The

REF19x will output a steady voltage to the AD7680.

Figure 6. Typical Connection Diagram

TBD

Digital Inputs

The digital inputs applied to the AD7680 are not limited

by the maximum ratings which limit the analog inputs.

Instead, the digital inputs applied can go to 7V and are

inputs. For example, if the AD7680 was operated with a

tal inputs. However, it is important to note that the data

output on SDATA will still have 3 V logic levels when

Another advantage of SCLK, and

CS not being restricted

sequencing issues are avoided. Ifone of these digital inputs

there would be on the analog inputs if a signal greater

MODES OF OPERATION

The mode of operation of the AD7680 is selected by con-

trolling the (logic) state of the

CS signal during a conver-

sion . There are two possible modes of operation, Normal

Mode and Power-Down Mode. The point at which

CS is

pulled high after the conversion has been initiated will

determine whether the AD7680 will enter Power-down

Mode or not. Similarly, if already in Power-down then

CS can control whether the device will return to Normal

operation or remain in Power-down. These modes of op-

eration are designed to provide flexible power manage-

ment options. These options can be chosen to optimize

the power dissipation/throughput rate ratio for differing

application requirements.