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AD7921

2-Channel, 2.35 V to 5.25 V 250 kSPS, 10-/12-Bit ADCs

Analog Devices 

AD7911/AD7921

TYPICAL CONNECTION DIAGRAM

Figure 22 shows a typical connection diagram for the AD7911/

AD7921. VREF is taken internally from VDD and as such VDD

should be well decoupled. This provides an analog input range

of 0 V to VDD. The conversion result is output in a 16-bit word

with two leading zeros, followed by the channel identifier bit

that identifies the channel converted, followed by an invalid bit

that matches up to the channel converted, followed by the MSB

of the 12-bit or 10-bit result. For the AD7911, the 10-bit result is

followed by two trailing zeros. See the Serial Interface section.

Alternatively, because the supply current required by the

AD7911/AD7921 is so low, a precision reference can be used as

the supply source to the AD7911/AD7921. A REF19x voltage

reference (REF195 for 5 V or REF193 for 3 V) can be used to

supply the required voltage to the ADC (see Figure 22). 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 (for example, 15 V). The REF19x outputs a

steady voltage to the AD7911/AD7921. If the low dropout

REF193 is used, the current it needs to supply to the AD7911/

AD7921 is typically 1.5 mA. When the ADC is converting at a

rate of 250 kSPS, the REF193 needs to supply a maximum of

2 mA to the AD7911/AD7921. The load regulation of the

REF193 is typically 10 ppm/mA (REF193, VS = 5 V), which

results in an error of 20 ppm (60 μV) for the 2 mA drawn from

it. This corresponds to a 0.082 LSB error for the AD7921 with

VDD = 3 V from the REF193 and a 0.061 LSB error for the

AD7911.

For applications where power consumption is a concern, the

power-down mode of the ADC and the sleep mode of the

REF19x reference should be used to improve power perform-

ance. See the Modes of Operation section.

1.5mA 0.1μF

3V

1μF

TANT

REF193

10μF

680nF

VDD

0V TO VDD

INPUT

VIN0

VIN1

GND

AD7911/ SCLK

AD7921 CS

DIN

DOUT

0.1μF

μC/μP

5V

SUPPLY

SERIAL

INTERFACE

Figure 22. REF193 as Power Supply to AD7911/AD7921

Table 6 provides some typical performance data with various

references used as a VDD source and a 50 kHz input tone under

the same setup conditions.

Table 6. AD7921 Performance for Various Voltage

References IC

Reference Tied to VDD

AD7921 SNR Performance (dB)

AD780 at 3 V

−73

REF193

−72.42

ADR433

−72.9

AD780 at 2.5 V

−72.86

REF192

−72.27

ADR421

−72.75

ANALOG INPUT

Figure 23 shows an equivalent circuit of the analog input

structure of the AD7911/AD7921. The two diodes, D1 and D2,

provide ESD protection for the analog input. Care must be

taken to ensure that the analog input signal never exceeds the

supply rails by more than 300 mV, because this would cause

these diodes to become forward biased and start conducting

current into the substrate. The maximum current these diodes

can conduct without causing irreversible damage to the part is

10 mA.

VDD

D1

C2

R1 20pF

VIN

C1

6pF

D2

CONVERSION PHASE—SWITCH OPEN

TRACK PHASE—SWITCH CLOSED

Figure 23. Equivalent Analog Input Circuit

The capacitor C1 in Figure 23 is typically about 6 pF and can

primarily be attributed to pin capacitance. The resistor R1 is a

lumped component made up of the on resistance of a track-

and-hold switch and also includes the on resistance of the input

multiplexer. This resistor is typically about 100 Ω. The capacitor

C2 is the ADC sampling capacitor and has a capacitance of

20 pF typically.

For ac applications, removing high frequency components from

the analog input signal is recommended using a band-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 can significantly affect the ac

performance of the ADC. This might necessitate the use of an

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

the particular application.

Rev. A | Page 16 of 28

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