AD7887ARM datasheet(12/24 Pages) AD

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AD7887

Rev. D | Page 12 of 24

ANALOG INPUT

Figure 12 shows an equivalent circuit of the analog input

structure of the AD7887. The two diodes, D1 and D2, provide

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

ensure that the analog input signal never exceed the supply rails

by more than 200 mV. Exceeding this value causes the diodes

to become forward biased and to start conducting into the

substrate. The maximum current these diodes can conduct

without causing irreversible damage to the part is 20 mA.

However, it is worth noting that a small amount of current

(1 mA) being conducted into the substrate due to an

overvoltage on an unselected channel can cause inaccurate

conversions on a selected channel. Capacitor C1 in Figure 12 is

typically about 4 pF and can primarily be attributed to pin

capacitance. Resistor R1 is a lumped component made up of the

on resistance of a multiplexer and a switch. This resistor is

typically about 100 Ω. Capacitor C2 is the ADC sampling

capacitor and typically has a capacitance of 20 pF.

Note that the analog input capacitance seen when in track mode

is typically 38 pF, whereas in hold mode it is typically 4 pF.

VIN

VDD

4pF

CONVERSION PHASE—SWITCH OPEN

TRACK PHASE—SWITCH CLOSED

20pF

Figure 12. Equivalent Analog Input Circuit

For ac applications, removing high frequency components from

the analog input signal is recommended by use of an RC

low-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 will 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 13 shows a graph of the total harmonic distortion vs. the

analog input signal frequency for different source impedances.

INPUT FREQUENCY (kHz)

–90

10.89

31.59

21.14

–85

–80

–75

–70

–65

49.86

THD vs. FREQUENCY FOR DIFFERENT

SOURCE IMPEDANCES

VDD = 5V

5V EXT REFERENCE

RIN = 1kΩ, CIN = 100pF

RIN = 50Ω, CIN = 2.2nF

RIN = 10Ω, CIN = 10nF

Figure 13. THD vs. Analog Input Frequency

On-Chip Reference

The AD7887 has an on-chip 2.5 V reference. This reference can

be enabled or disabled by clearing or setting the REF bit in the

control register, respectively. If the on-chip reference is to be used

externally in a system, it must be buffered before it is applied

elsewhere. If an external reference is applied to the device, the

internal reference is automatically overdriven. However, it is

advised to disable the internal reference by setting the REF bit

in the control register when an external reference is applied in

order to obtain optimum performance from the device. When

the internal reference is disabled, SW1, shown in Figure 14,

opens and the input impedance seen at the AIN1/VREF pin is the

input impedance of the reference buffer, which is in the region

of gigaohms. When the internal reference is enabled, the input

impedance seen at the pin is typically 10 kΩ. When the AD7887

is operated in two-channel mode, the reference is taken from

VDD internally, not from the on-chip 2.5 V reference.

2.5V

10kΩ

SW1

AIN1/VREF

Figure 14. On-Chip Reference Circuitry

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