AD7091 데이터 시트보기 (PDF) - Analog Devices

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AD7091

1 MSPS, Ultralow Power, 12-Bit ADC in 8-Lead LFCSP

Analog Devices 

Data Sheet

TYPICAL CONNECTION DIAGRAM

Figure 19 shows a typical connection diagram for the AD7091.

A positive power supply in the range of 2.09 V to 5.25 V should

be connected to the VDD pin. The reference is derived internally

from VDD and, for this reason, VDD should be well decoupled to

achieve the specified performance; typical values for the decoupling

capacitors are 100 nF and 10 µF. The analog input range is 0 V

to VDD. The typical value for the regulator bypass decoupling

capacitor (REGCAP) is 1 µF. The conversion result is output in

a 12-bit word with the MSB first.

Alternatively, because the supply current required by the AD7091

is so low, a precision reference can be used as the supply source

to the part. A reference such as the REF195 or ADR4550 can be

used where a 5 V supply is desired. The REF193 or ADR4530

are recommended for use when a 3 V supply is required for the

ADC. This configuration is especially useful if the power supply

is quite noisy, or if the system supply voltages are at some value

other than 5 V or 3 V, such as 15 V.

If the busy indicator function is required, connect a pull-up

resistor of typically 100 kΩ to VDD to the SDO pin (see Figure 19).

In addition, for applications in which power consumption is

a concern, the power-down mode can be used to improve the

power performance of the ADC (see the Modes of Operation

section for more information).

ANALOG INPUT

Figure 18 shows an equivalent circuit of the AD7091 analog

input structure. The D1 and D2 diodes provide ESD protection

for the analog input. To prevent the diodes from becoming

forward-biased and conducting current, ensure that the analog

input signal never exceeds VDD by more than 300 mV. These diodes

can conduct a maximum of 10 mA without causing irreversible

damage to the part.

VDD

D1

VIN

C1

1pF

D2

C2

R1 3.6pF

C3

2.5pF

AD7091

NOTES

1. DURING THE CONVERSION PHASE, THE SWITCH IS OPEN.

DURING THE TRACK PHASE, THE SWITCH IS CLOSED.

Figure 18. Equivalent Analog Input Circuit

Capacitor C1 in Figure 18 is typically about 1 pF and can

primarily be attributed to pin capacitance. Resistor R1 is a

lumped component made up of the on resistance of a switch.

This resistor is typically about 500 Ω. Capacitor C2 is the ADC

sampling capacitor and typically has a capacitance of 3.6 pF.

In applications where harmonic distortion and signal-to-noise

ratio (SNR) are critical, the analog input should be driven from

a low impedance source. Large source impedances significantly

affect the ac performance of the ADC and may necessitate the

use of an input buffer amplifier, as shown in Figure 19. 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. Figure 9 shows

a graph of THD vs. source impedance when using a supply

voltage of 3 V and a sampling rate of 1 MSPS.

To achieve the specified performance, use an external filter—such

as the one-pole, low-pass RC filter shown in Figure 19—on the

analog input connected to the AD7091.

VDD

(2.09V to 5.25V)

10μF 100nF

1μF

WITH BUSY

INDICATOR

VDD

100kΩ

ANALOG

INPUT

51Ω

VIN

4.7nF

VDD

REGCAP

SDO

AD7091

SCLK

CS

GND

CONVST

MICROPROCESSOR/

MICROCONTROLLER/

DSP

Figure 19. Typical Connection Diagram

Rev. B | Page 11 of 20

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