AD7298BCPZ(RevPrA) データシートの表示（PDF） - Analog Devices

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AD7298BCPZ
(Rev.:RevPrA)

8-Channel, 1MSPS, 12-Bit SAR ADC with Temperature Sensor

Analog Devices 

Preliminary Technical Data

AD7298

CIRCUIT INFORMATION

The AD7298 is a high speed, 8-channel, 12-bit, ADC with

internal temperature sensor. The part can be operated from a

2.8 V to 3.6 V supply and is capable of throughput rates of

1MSPS per analog input channel.

The AD7298 provides the user with an on-chip, track-and-hold

ADC and a serial interface housed in a 20-lead LFCSP. The

AD7298 has eight single-ended input channels with channel

repeat functionality, which allows the user to select a channel

sequence through which the ADC can cycle with each

consecutive CS falling edge. The serial clock input accesses data

from the part, controls the transfer of data written to the ADC,

and provides the clock source for the successive approximation

ADC. The analog input range for the AD7928 is 0V to VREF. The

AD7298 operates with one cycle latency, which means that

conversion result is available in the serial transfer following the

cycle in which the conversion is performed.

The AD7298 includes a high accuracy band-gap temperature

sensor, which is monitored and digitized by the 12-bit ADC to

give a resolution of 0.25°C. The AD7298 provides flexible power

management options to allow the user to achieve the best power

performance for a given throughput rate. These options are

selected by programming the power-down bit, PD, in the

control register.

CONVERTER OPERATION

The AD7298 is a 12-bit successive approximation ADC based

around a capacitive DAC. Figure 8 and Figure 9 show simplified

schematics of the ADC. The ADC is comprised of control logic,

SAR, and a capacitive DAC that are used to add and subtract

fixed amounts of charge from the sampling capacitor to bring

the comparator back into a balanced condition. Figure 8 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.

comparator is rebalanced, the conversion is complete. The

control logic generates the ADC output code. Figure 11 shows

the ADC’s transfer functions.

Figure 9. ADC Conversion Phase

ANALOG INPUT

Figure 11 shows an equivalent circuit of the analog input struc-

ture of the AD7298. 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 internally

generated LDO voltage of 2.5V (DCAP) by more than 300 mV.

This causes the diodes to become forward biased and start

conducting current into the substrate. 10 mA is the maximum

current these diodes can conduct without causing irreversible

damage to the part. Capacitor C1, in Figure 10 is typically about

TBD pF and can primarily be attributed to pin capacitance. The

Resistor R1 is a lumped component made up of the on

resistance of a switch (track-and-hold switch) and also includes

the on resistance of the input multiplexer. The total resistance is

typically about TBD Ω. The capacitor, C2, is the ADC sampling

capacitor and has a capacitance of TBD pF typically.

Figure 10. Equivalent Analog Input Circuit

Figure 8. ADC Acquisition Phase

When the ADC starts a conversion (see Figure 9), SW2

opens and SW1 moves to Position B, causing the comparator

to become unbalanced. The control logic and the capacitive

DAC are used to add and subtract fixed amounts of charge to

bring the comparator back into a balanced condition. When the

For AC applications, removing high frequency components

from the analog input signal is recommended by using

an RC low-pass filter on the relevant analog input pin. In

applications where harmonic distortion and signal-to-noise

ratios 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 performance criteria.

Rev. PrA | Page 13 of 18

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