AD7854ARS datasheet(14/28 Pages) AD

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AD7854/AD7854L

–14–

REV. B

INPUT FREQUENCY – kHz

–72

–92

100

–76

–80

–84

–88

RIN = 1k

RIN = 50 , 10nF

AS IN FIGURE 13

THD VS. FREQUENCY FOR DIFFERENT

SOURCE IMPEDANCES

Figure 10. THD vs. Analog Input Frequency

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. Figure 10

shows a graph of the total harmonic distortion vs. analog input

signal frequency for different source impedances. With the

setup as in Figure 11, the THD is at the –90 dB level. With a

source impedance of 1 k

Ω and no capacitor on the AIN(+) pin,

the THD increases with frequency.

In a single supply application (both 3 V and 5 V), the V+ and

V– of the op amp can be taken directly from the supplies to the

AD7854/AD7854L which eliminates the need for extra external

power supplies. When operating with rail-to-rail inputs and out-

puts at frequencies greater than 10 kHz, care must be taken in

selecting the particular op amp for the application. In particular,

for single supply applications the input amplifiers should be

connected in a gain of –1 arrangement to get the optimum per-

formance. Figure 11 shows the arrangement for a single supply

application with a 50

Ω and 10 nF low-pass filter (cutoff fre-

quency 320 kHz) on the AIN(+) pin. Note that the 10 nF is a

capacitor with good linearity to ensure good ac performance.

Recommended single supply op amps are the AD820 and the

AD820-3V.

V–

V+

10k

VIN

(–VREF/2 TO +VREF/2)

VREF/2

0.1 F

10 F

10nF

(NPO)

AD820

AD820-3V

TO AIN(+) OF

AD7854/AD7854L

+3V TO +5V

10k

IC1

10k

Figure 11. Analog Input Buffering

ANALOG INPUT

The equivalent analog input circuit is shown in Figure 9. Dur-

ing the acquisition interval the switches are both in the track

position and the AIN(+) charges the 20 pF capacitor through

the 125

Ω resistance. On the rising edge of CONVST switches

SW1 and SW2 go into the hold position retaining charge on the

20 pF capacitor as a sample of the signal on AIN(+). The

AIN(–) is connected to the 20 pF capacitor, and this unbalances

the voltage at Node A at the input of the comparator. The

capacitor DAC adjusts during the remainder of the conversion

cycle to restore the voltage at Node A to the correct value. This

action transfers a charge, representing the analog input signal, to

the capacitor DAC which in turn forms a digital representation

of the analog input signal. The voltage on the AIN(–) pin directly

influences the charge transferred to the capacitor DAC at the

hold instant. If this voltage changes during the conversion period,

the DAC representation of the analog input voltage is altered.

Therefore it is most important that the voltage on the AIN(–)

pin remains constant during the conversion period. Further-

more, it is recommended that the AIN(–) pin is always connected

to AGND or to a fixed dc voltage.

CAPACITOR

DAC

COMPARATOR

20pF

HOLD

TRACK

SW2

TRACK

SW1

HOLD

125

AIN(+)

125

AIN(–)

AGND

NODE A

Figure 9. Analog Input Equivalent Circuit

Acquisition Time

The track-and-hold amplifier enters its tracking mode on the

falling edge of the BUSY signal. The time required for the

track-and-hold amplifier to acquire an input signal depends on

how quickly the 20 pF input capacitance is charged. There is a

minimum acquisition time of 400 ns. For large source imped-

ances, >2 k

Ω, the acquisition time is calculated using the formula:

tACQ = 9

× (RIN + 125 Ω) × 20 pF

where RIN is the source impedance of the input signal, and

125

Ω, 20 pF is the input R, C.

DC/AC Applications

For dc applications, high source impedances are acceptable,

provided there is enough acquisition time between conversions

to charge the 20 pF capacitor. For example with RIN = 5 k

Ω,

the required acquisition time is 922 ns.

For ac applications, removing high frequency components from

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

pass filter on the AIN(+) pin, as shown in Figure 11. In applica-

tions where harmonic distortion and signal to noise ratio are

critical, the analog input should be driven from a low impedance

source. Large source impedances significantly affect the ac per-

formance of the ADC. They may require the use of an input

buffer amplifier. The choice of the amplifier is a function of the

particular application.

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