AD7450ARMZ datasheet(13/23 Pages) AD

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–12–

AD7450

REFERENCE = 1.25V

1.25V p-p

COMMON-MODE (CM)

CMMIN = 0.625V

CMMAX = 4.42V

REFERENCE = 2.5V

2.5V p-p

COMMON-MODE (CM)

CMMIN = 1.25V

CMMAX = 3.75V

VIN–

VIN

VIN–

VIN

Figure 10. Examples of the Analog Inputs to VIN+

and VIN– for Different Values of VREF for VDD = 5 V

Analog Input Structure

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

ture of the AD7450. 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 will cause 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 11 are typically 4 pF and can prima-

rily 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.

For ac applications, removing high-frequency components from

the analog input signal is recommended by the use of an RC

low-pass filter on the relevant analog input pins. 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 will significantly affect the ac perfor-

mance of the ADC. This may necessitate the use of an input

buffer amplifier. The choice of the op amp will be a function of

the particular application.

VDD

VIN+

VIN–

VDD

Figure 11. Equivalent Analog Input Circuit

Conversion Phase—Switches Open

Track Phase—Switches Closed

When no amplifier is used to drive the analog input, the source

impedance should be limited to values lower than 1 k

Ω. The

maximum source impedance will depend on the amount of

total harmonic distortion (THD) that can be tolerated. The THD

will increase as the source impedance increases and the perfor-

mance will degrade. Figure 12 shows a graph of the THD versus

the analog input signal frequency for different source impedances.

INPUT FREQUENCY – kHz

–74

–70

–72

–76

–78

–82

–80

1000

100

TA = 25 C

VDD = 5V

RIN = 1k

VDD = 3V

RIN = 1k

VDD = 3V

RIN = 100

VDD = 5V

RIN = 100

Figure 12. THD vs. Analog Input Frequency for

Various Source Impedances for VDD = 5 V and 3 V

Figure 13 shows a graph of the THD versus the analog input

frequency for VDD of 5 V

± 5% and 3 V ± 10%, while sampling

at 1 MSPS and 833 kSPS with a SCLK of 18 MHz and

15 MHz, respectively. In this case, the source impedance is 10

Ω.

INPUT FREQUENCY – kHz

–95

–70

–60

–65

–75

–80

–90

–85

1000

100

TA = 25 C

VDD = 3.3V

VDD = 2.7V

VDD = 4.75V

VDD = 5.25V

Figure 13. THD vs. Analog Input Frequency for 3 V

± 10% and 5 V ± 5% Supply Voltages

DRIVING DIFFERENTIAL INPUTS

Differential operation requires that VIN+ and VIN– be simulta-

neously driven with two equal signals that are 180

o out of phase.

The common mode must be set up externally and has a range

that is determined by VREF, the power supply, and the particular

amplifier used to drive the analog inputs (see Figures 8 and 9).

Differential modes of operation with either an ac or dc input

provide the best THD performance over a wide frequency range.

Since not all applications have a signal preconditioned for

differential operation, there is often a need to perform single-

ended-to-differential conversion.

Rev. A

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