REV. E

AD7701

–11–

INPUT SIGNAL CONDITIONING

Reference voltages from 1 V to 3 V may be used with the AD7701

with little degradation in performance. Input ranges that cannot

be accommodated by this range of reference voltages may be

achieved by input signal conditioning. This may take the form

of gain to accommodate a smaller signal range, or passive attenua-

tion to reduce a larger input voltage range.

Source Resistance

If passive attenuators are used in front of the AD7701, care must

be taken to ensure that the source impedance is sufficiently low.

The AD7701 has an analog input with over 1 G

Ω dc input

resistance. In parallel with this, there is a small dynamic load that

varies with the clock frequency (see Figure 13). Each time the

analog input is sampled, a 10 pF capacitor draws a charge packet

of maximum 1 pC (10 pF

× 100 mV) from the analog source

R1

R2

AGND

AD7701

10pF

Figure 13. Equivalent Input Circuit and Input Attenuator

yields an average current draw of 16 nA. After each sample, the

AD7701 allows 62 clock periods for the input voltage to settle.

The equation that defines settling time is:

VV

e

OIN

tRC

=−

[]

−

1

where

R is the value of the input source resistance.

C is the 10 pF sample capacitor.

From this, the following equation can be developed, which

=

62

×(10 pF)× ln(100mV /V

Provided the source resistance is less than this value, the analog

input will settle within the desired error band in the requisite 62

clock periods. Insufficient settling leads to offset errors. These

can be calibrated in system calibration schemes.

If a limit of 10

µV (0.25 LSB at 16 bits) is set for the maximum

offset voltage, then the maximum allowable source resistance is

160 k

Ω from the above equation, assuming that there is no

external stray capacitance.

An RC filter may be added in front of the AD7701 to reduce

high frequency noise. With an external capacitor added from

allowable source resistance:

62

100 mV

× C

V

E









The practical limit to the maximum value of source resistance is

thermal (Johnson) noise. A practical resistor may be modeled as

an ideal (noiseless) resistor in series with a noise voltage source

or in parallel with a noise current source:

V

kTRf Volts

n

= 4

i

kTRf R Amperes

n

= 4

where

k is Boltzmann’s constant (1.38

T is temperature in degrees Kelvin (

°C + 273).

Active signal conditioning circuits such as op amps generally do

not suffer from problems of high source impedance. Their open-

loop output resistance is normally only tens of ohms and, in any

case, most modern general-purpose op amps have sufficiently

fast closed-loop settling time for this not to be a problem. Offset

voltage in op amps can be eliminated in a system calibration

routine. With the wide dynamic range and small LSB size of the

AD7701, noise can also be a problem, but the digital filter will

reject most broadband noise above its cutoff frequency. How-

ever, in certain applications there may be a need for analog

input filtering.

Antialias Considerations

The digital filter of the AD7701 does not provide any rejection

where n = 1, 2, 3 . . . ).

With a 4.096 MHz master clock, there are narrow (

± 10 Hz)

bands at 16 kHz, 32 kHz, 48 kHz, and so on, where noise

passes unattenuated to the output.

However, due to the AD7701’s high oversampling ratio of 800

(16 kHz to 20 Hz), these bands occupy only a small fraction of

the spectrum and most broadband noise is filtered. The reduc-

tion in broadband noise is given by:

= e

IN

= 0.035 e

IN

where

broadband white noise by 96.5% independent of the master

clock frequency.