AD7440BRT-R2 datasheet(17/28 Pages) AD

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AD7440/AD7450A

Rev. B | Page 17 of 28

Figure 30 shows examples of the inputs to VIN+ and VIN– for

different values of VREF for VDD = 5 V. It also gives the maximum

and minimum common-mode voltages for each reference value

according to Figure 28.

COMMON-MODE (CM)

CMMIN = 1V

CMMAX = 4V

REFERENCE = 2V

VIN–

VIN+

2V p-p

COMMON-MODE (CM)

CMMIN = 1.25V

CMMAX = 3.75V

REFERENCE = 2.5V

VIN–

VIN+

2.5V p-p

Figure 30. Examples of the Analog Inputs to VIN+ and VIN– for

Different Values of VREF for VDD = 5 V

Analog Input Structure

Figure 31 shows the equivalent circuit of the analog input

structure of the AD7440/AD7450A. The four diodes provide

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

ensure that the analog input signals never exceed the supply

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

forward biased and start conducting into the substrate. These

diodes can conduct up to 10 mA without causing irreversible

damage to the part. The capacitors, C1 in Figure 31, are typically

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

resistors are lumped components made up of the on resistance

of the switches. The value of these resistors is typically about

100 Ω. The capacitors, C2, are the ADC’s sampling capacitors

and have a capacitance of 16 pF typically.

VDD

VIN+

VIN–

Figure 31. Equivalent Analog Input Circuit

Conversion Phase–Switches Open; Track Phase–Switches Closed

For ac applications, removing high frequency components from

the analog input signal through the use of an RC low-pass filter

on the relevant analog input pins is recommended. In applica-

tions where harmonic distortion and signal-to-noise ratio are

critical, the analog input should be driven from a low impe-

dance 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 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 32 shows a graph of THD versus the analog input signal

frequency for different source impedances for VDD = 5 V.

–100

–80

–60

–40

–20

100

1000

INPUT FREQUENCY (kHz)

TA = 25°C

VDD = 5V

RIN = 1kΩ

RIN = 510Ω

RIN = 10Ω

RIN = 300Ω

Figure 32. THD vs. Analog Input Frequency for Various Source Impedances

for VDD=5V

Figure 33 shows a graph of the THD versus the analog input

frequency for VDD of 5 V ± 5% and 3 V + 20%/–10%, while

sampling at 1 MSPS with an SCLK of 18 MHz. In this case, the

source impedance is 10 Ω.

–50

–90

–85

–80

–75

–70

–65

–60

–55

100

1000

INPUT FREQUENCY (kHz)

TA = 25°C

VDD = 2.7V

VDD = 3.6V

VDD = 5.25V

VDD = 4.75V

Figure 33. THD vs. Analog Input Frequency for 3 V and 5 V Supply Voltages

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