AD7680BRJZ-R2 datasheet(13/25 Pages) AD

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AD7680

Rev. A | Page 12 of 24

CIRCUIT INFORMATION

The AD7680 is a fast, low power, 16-bit, single-supply ADC. The

part can be operated from a 2.5 V to 5.5 V supply and is capable of

throughput rates of 100 kSPS when provided with a 2.5 MHz clock.

The AD7680 provides the user with an on-chip track-and-hold

ADC and a serial interface housed in a tiny 6-lead SOT-23

package or in an 8-lead MSOP package, which offer the user

considerable space-saving advantages over alternative solutions.

The serial clock input accesses data from the part and also

provides the clock source for the successive approximation

ADC. The analog input range for the AD7680 is 0 V to VDD. An

external reference is not required for the ADC nor is there a

reference on-chip. The reference for the AD7680 is derived from

the power supply and thus gives the widest dynamic input range.

The AD7680 also features a power-down option to save power

between conversions. The power-down feature is implemented

across the standard serial interface as described in the Modes of

Operation section.

CONVERTER OPERATION

The AD7680 is a 16-bit, successive approximation ADC based

around a capacitive DAC. The AD7680 can convert analog

input signals in the 0 V to VDD range. Figure 11 and Figure 12

show simplified schematics of the ADC. The ADC comprises

control logic, SAR, and a capacitive DAC. Figure 11 shows the

ADC during its acquisition phase. SW2 is closed and SW1 is in

Position A. The comparator is held in a balanced condition and

the sampling capacitor acquires the signal on the selected VIN

channel.

CAPACITIVE

DAC

CONTROL

LOGIC

SAMPLING

CAPACITOR

COMPARATOR

ACQUISITION

PHASE

SW1

VDD/2

SW2

VIN

Figure 11. ADC Acquisition Phase

When the ADC starts a conversion, SW2 opens and SW1 moves

to Position B, causing the comparator to become unbalanced

(Figure 12). The control logic and the capacitive DAC are used

to add and subtract fixed amounts of charge from the sampling

capacitor to bring the comparator back into a balanced

condition. When the comparator is rebalanced, the conversion

is complete. The control logic generates the ADC output code

(see the ADC Transfer Function section).

CAPACITIVE

DAC

CONTROL

LOGIC

SAMPLING

CAPACITOR

COMPARATOR

CONVERSION

PHASE

SW1

VDD/2

SW2

VIN

Figure 12. ADC Conversion Phase

ANALOG INPUT

Figure 13 shows an equivalent circuit of the analog input

structure of the AD7680. The two diodes, D1 and D2, provide

ESD protection for the analog inputs. Care must be taken to

ensure that the analog input signal never exceeds the supply

rails by more than 300 mV. This causes these diodes to become

forward-biased and to start conducting current into the

substrate. The maximum current these diodes can conduct

without causing irreversible damage to the part is 10 mA.

Capacitor C1 in Figure 13 is typically about 5 pF and can be

attributed primarily to pin capacitance. Resistor R1 is a lumped

component made up of the on resistance of a track-and-hold

switch. This resistor is typically about 25 Ω. Capacitor C2 is the

ADC sampling capacitor and has a capacitance of 25 pF

typically. For ac applications, removing 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 harmonic distortion and signal-to-noise

ratio are critical, the analog input should be driven from a low

impedance source. Large source impedances significantly affect

the ac performance of the ADC. This may necessitate the use of

an input buffer amplifier. The choice of the op amp is a function

of the particular application. When no amplifier is used to drive

the analog input, the source impedance should be limited to low

values. The maximum source impedance depends on the

amount of total harmonic distortion (THD) that can be

tolerated. The THD increases as the source impedance

increases, and performance degrades (see Figure 8).

25pF

CONVERSION PHASE - SWITCH OPEN

TRACK PHASE - SWITCH CLOSED

VIN

VDD

5pF

Figure 13. Equivalent Analog Input Circuit

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