AD7986 datasheet(16/29 Pages) AD

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Data Sheet

AD7986

Rev. D | Page 15 of 28

ANALOG INPUTS

Figure 24 shows an equivalent circuit of the input structure of

the AD7986.

The two diodes, D1 and D2, provide ESD protection for the analog

inputs, IN+ and IN−. Take care to ensure the analog input signal

does not exceed the reference input voltage (REF) by more than

0.3 V. If the analog input signal exceeds this level, the diodes

become forward-biased and start conducting current. These

diodes can handle a forward-biased current of 130 mA maximum.

However, if the supplies of the input buffer (for example, the V+

and V− supplies of the buffer amplifier in Figure 23) are different

from those of REF, the analog input signal may eventually exceed

the supply rails by more than 0.3 V. In such a case (for example,

an input buffer with a short circuit), the current limitation can

protect the device.

CPIN

REF

RIN

CIN

IN+ OR IN–

GND

Figure 24. Equivalent Analog Input Circuit

The analog input structure allows the sampling of the true

differential signal between IN+ and IN−. By using these differential

inputs, signals common to both inputs are rejected.

During the acquisition phase, the impedance of the analog

inputs (IN+ or IN−) can be modeled as a parallel combination

of Capacitor CPIN and the network formed by the series connection

of Resistor RIN and Capacitor CIN. Capacitor CPIN is primarily the

pin capacitance. Resistor RIN is typically 400 Ω and is a lumped

component composed of serial resistors and the on resistance of

the switches. Capacitor CIN is typically 30 pF and is mainly the

ADC sampling capacitor.

During the sampling phase, where the switches are closed, the

input impedance is limited to Capacitor CPIN. Resistor RIN and

Capacitor CIN make a one-pole, low-pass filter that reduces

undesirable aliasing effects and limits noise.

When the source impedance of the driving circuit is low, the

AD7986 can be driven directly. Large source impedances

significantly affect the ac performance, especially THD. The

dc performances are less sensitive to the input impedance. The

maximum source impedance depends on the amount of THD

that can be tolerated. The THD degrades as a function of the

source impedance and the maximum input frequency.

DRIVER AMPLIFIER CHOICE

Although the AD7986 is easy to drive, the driver amplifier must

meet the following requirements:

•

The noise generated by the driver amplifier must be kept as

low as possible to preserve the SNR and transition noise

performance of the AD7986. The noise from the driver is

filtered by the AD7986 analog input circuit one-pole, low-

pass filter, made by RIN and CIN or by the external filter if

one is used. Because the typical noise of the AD7986 is

44 µV rms, the SNR degradation due to the amplifier is













−

)

(

log

LOSS

SNR

where:

f–3dB is the input bandwidth, in megahertz, of the AD7986

(19 MHz) or the cutoff frequency of the input filter, if

one is used.

N is the noise gain of the amplifier (for example, 1 in buffer

configuration).

eN is the equivalent input noise voltage of the operational

amplifier, in nV/√Hz.

•

For ac applications, the driver must have a THD perfor-

mance commensurate with the AD7986.

•

For multichannel multiplexed applications, the driver

amplifier and the AD7986 analog input circuit must settle

for a full-scale step onto the capacitor array at an 18-bit level

(0.0004%, 4 ppm). In the data sheet of the driver amplifier,

settling at 0.1% to 0.01% is more commonly specified. This

may differ significantly from the settling time at an 18-bit

level and must be verified prior to driver selection.

Table 8. Recommended Driver Amplifiers

Amplifier

Typical Application

AD8021

Very low noise and high frequency

AD8022

Low noise and high frequency

ADA4899-1

Ultralow noise and high frequency

AD8014

Low power and high frequency

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