AD8331ARQZ-R7 datasheet(29/40 Pages) AD

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AD8331/AD8332/AD8334

Rev. E | Page 29 of 40

Noise

Output clamping can be used for ADC input overload

protection, if needed, or postamp overload protection when

operating from a lower common-mode level, such as 1.5 V. The

user should be aware that distortion products increase as output

levels approach the clamping levels, and the user should adjust

the clamp resistor accordingly. For additional information, see

the Applications section.

The topology of the postamplifier provides constant input-

referred noise with the two gain settings and variable

output-referred noise. The output-referred noise in HI gain

mode increases (with gain) by four. This setting is recommended

when driving converters with higher noise floors. The extra gain

boosts the output signal levels and noise floor appropriately. When

driving circuits with lower input noise floors, the LO gain mode

optimizes the output dynamic range.

The accuracy of the clamping levels is approximately ±5% in LO

or HI mode. Figure 80 illustrates the output characteristics for a

few values of RCLMP.

Although the quantization noise floor of an ADC depends on a

number of factors, the 48 nV/√Hz and 178 nV/√Hz levels are

well suited to the average requirements of most 12-bit and 10-bit

converters, respectively. An additional technique, described in

the Applications section, can extend the noise floor even lower

for possible use with 14-bit ADCs.

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

–3

–2

–1

VINH (V)

RCLMP = ∞

8.8kΩ

3.5kΩ

RCLMP = 1.86kΩ

Output Clamping

Outputs are internally limited to a level of 4.5 V p-p differential

when operating at a 2.5 V common-mode voltage. The postamp

implements an optional output clamp engaged through a

resistor from RCLMP to ground. Table 8 shows a list of

recommended resistor values.

Figure 80. Output Clamping Characteristics

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