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AD4000/AD4004/AD4008

Data Sheet

Rev. C | Page 18 of 36

CONVERTER OPERATION

The AD4000/AD4004/AD4008 are SAR-based ADCs using a

charge redistribution sampling digital-to-analog converter

(DAC). Figure 31 shows the simplified schematic of the ADC.

The capacitive DAC consists of two identical arrays of 16 binary

weighted capacitors, which are connected to the comparator

inputs.

During the acquisition phase, terminals of the array tied to the

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

and SW− switches. All independent switches connect the other

terminal of each capacitor to the analog inputs. The capacitor

arrays are used as sampling capacitors and acquire the analog

signal on the IN+ and IN− inputs.

When the acquisition phase is complete and the CNV input

goes high, a conversion phase initiates. 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. The differential voltage between the IN+ and

IN− inputs 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 VREF, the comparator input varies by

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

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

to bring the comparator back into a balanced condition. After

the completion of this process, the control logic generates the

ADC output code and a busy signal indicator.

Because the AD4000, the AD4004, and the AD4008 have on-

board conversion clocks, the serial clock, SCK, is not required

for the conversion process.

TRANSFER FUNCTIONS

The ideal transfer characteristics for the AD4000/AD4004/AD4008

are shown in Figure 32 and Table 9.

000...000

000...001

000...010

111...101

111...110

111...111

ANALOG INPUT

+FSR – 1.5 LSB

+FSR – 1 LSB

–FSR + 1 LSB

–FSR

–FSR + 0.5 LSB

Figure 32. ADC Ideal Transfer Function (FSR Is Full-Scale Range)

Table 9. Output Codes and Ideal Input Voltages

Description

Analog Input, VREF = 5 V

VREF = 5 V with Span Compression Enabled (V)

Digital Output Code

FSR − 1 LSB

4.999924 V

4.499939

0xFFFF1

Midscale + 1 LSB

2.500076 V

2.500061

0x8001

Midscale

2.5 V

2.5

0x8000

Midscale − 1 LSB

2.499924 V

2.499939

0x7FFF

−FSR + 1 LSB

76.3 µV

0.50006103

0x0001

−FSR

0 V

0.5

0x00002

This output code is also the code for an overranged analog input (VIN+ − VIN− above VREF with span compression disabled and above 0.9 × VREF with span compression

enabled).

This output code is also the code for an underranged analog input (VIN+ − VIN− below 0 V with span compression disabled and below 0.1 × VREF with span compression

enabled).

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