AD7091 datasheet(11/20 Pages) AD

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Data Sheet

AD7091

Rev. A | Page 11 of 20

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 from the REF19x or ADR34xx voltage

reference family (REF195 or ADR3450 for 5 V, and REF193 or

ADR3430 for 3 V) can be used to supply the required voltage

to 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.

3.6pF

VDD

VIN

1pF

2.5pF

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.

AD7091

VIN

GND

CONVST

SDO

VDD

SCLK

REGCAP

MICROPROCESSOR/

MICROCONTROLLER/

DSP

10μF

100nF

1μF

100kΩ

WITH BUSY

INDICATOR

ANALOG

INPUT

VDD

51Ω

4.7nF

(2.09V to 5.25V)

Figure 19. Typical Connection Diagram

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