AD7674ASTZ datasheet(19/29 Pages) AD

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AD7674

Data Sheet

Rev. B | Page 18 of 28

Analog Inputs

Figure 29 shows a simplified analog input section of the AD7674.

The diodes shown in Figure 29 provide ESD protection for the

inputs. Care must be taken to ensure that the analog input signal

never exceeds the absolute ratings on these inputs. This causes

these diodes to become forward biased and start conducting

current. These diodes can handle a forward-biased current of

120 mA max. This condition can eventually occur when the U1

or U2 supplies of the input buffer are different from AVDD. In

such a case, an input buffer with a short-circuit current

limitation can be used to protect the device.

IN+

IN–

AGND

AVDD

R+ = 102

Ω

R– = 102

Ω

03083-0-028

Figure 29. Simplified Analog Input

This analog input structure is a true differential structure. By

using these differential inputs, signals common to both inputs

are rejected as shown in Figure 30, which represents typical

CMRR over frequency.

FREQUECY (kHz)

100

1000

10000

03083-0-029

Figure 30. Analog Input CMRR vs. Frequency

During the acquisition phase for ac signals, the AD7674 behaves

like a 1-pole RC filter consisting of the equivalent resistance R+,

R–, and CS. The R+ and R– resistors are typically 102 Ω and are

lumped components made up of a serial resistor and the on

resistance of the switches. CS is typically 60 pF and mainly

consists of the ADC sampling capacitor. This 1-pole filter with a

−3 dB cutoff frequency of 26 MHz typ reduces any undesirable

aliasing effect and limits the noise coming from the inputs.

Because the input impedance of the AD7674 is very high, the

device can be driven directly by a low impedance source without

gain error. This allows the user to put an external 1-pole RC

filter between the amplifier output and the ADC analog inputs,

as shown in Figure 28, to improve the noise filtering done by the

AD7674 analog input circuit. However, the source impedance has

to be kept low because it affects the ac performance, especially

the total harmonic distortion (THD). The maximum source

impedance depends on the amount of THD that can be tolerated.

The THD degrades as a function of source impedance and the

maximum input frequency, as shown in Figure 31.

INPUT RESISTANCE (

Ω)

–95

–120

105

–100

–105

–110

–115

20kHz

10kHz

2kHz

03083-0-030

Figure 31. THD vs. Analog Input Frequency and Source Resistance

Driver Amplifier Choice

Although the AD7674 is easy to drive, the driver amplifier

needs to meet the following requirements:

•

The driver amplifier and the AD7674 analog input circuit

have to be able to settle for a full-scale step of the capacitor

array at an 18-bit level (0.0004%). In the amplifier data sheet,

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

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

level and, therefore, should be verified prior to driver

selection. The tiny op amp AD8021, which combines

ultralow noise and high gain-bandwidth, meets this

settling time requirement.

•

The noise generated by the driver amplifier needs to be

kept as low as possible to preserve the SNR and transition

noise performance of the AD7674. The noise coming from

the driver is filtered by the AD7674 analog input circuit 1-

pole low-pass filter made by R+, R–, and CS. The SNR

degradation due to the amplifier is

















)

(

625

log

3dB

–

LOSS

SNR

where:

f–3dB is the –3 dB input bandwidth in MHz of the AD7674

(26 MHz) or the cutoff frequency of the input filter, if used.

N is the noise factor of the amplifiers (1 if in buffer

configuration).

eN is the equivalent input noise voltage of each op amp in

nV/√Hz.

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