AD8331 datasheet(24/32 Pages) AD

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AD8331/AD8332

Rev. C | Page 24 of 32

Table 4. Clamp Resistor Values

Clamp Resistor Value (kΩ)

Clamp Level

(V p-p)

HILO = LO

HILO = HI

0.5

1.21

1.0

2.74

2.21

1.5

4.75

4.02

2.0

7.5

6.49

2.5

9.53

3.0

16.9

14.7

3.5

26.7

23.2

4.0

49.9

39.2

4.4

100

73.2

Output Filtering and Series Resistor

Requirements

To ensure stability at the high end of the gain control range,

series resistors or ferrite beads are recommended for the

outputs when driving large capacitive loads, or circuits on other

boards,. These components can be part of the external

noise filter.

Recommended resistor values are 84.5 Ω for LO gain mode and

100 Ω for HI gain mode (see Figure 66) and are placed near

Pins VOH and VOL. Lower value resistors are permissible for

applications with nearby loads or with gains less than 40 dB.

Lower values are best selected empirically.

An antialiasing noise filter is typically used with an ADC. Filter

requirements are application dependent.

When the ADC resides on a separate board, the majority of

filter components should be placed nearby to suppress noise

picked up between boards and mitigates charge kickback from

the ADC inputs. Any series resistance beyond that required for

output stability should be placed on the ADC board. Figure 70

shows a second order low-pass filter with a bandwidth of

20 MHz. The capacitor is chosen in conjunction with the 10 pF

input capacitance of the ADC.

18pF

OPTIONAL

BACKPLANE

84.5

Ω

0.1

µF

0.1

µF

1.5

µH

1.5

µH

158

Ω

158

Ω

84.5

Ω

ADC

Figure 70. 20 MHz Second-Order Low-Pass Filter

DRIVING ADCS

The output drive will accommodate a wide range of ADCs. The

noise floor requirements of the VGA will depend on a number

of application factors, including bit resolution, sampling rate,

full-scale voltage, and the bandwidth of the noise/antialias filter.

The output noise floor and gain range can be adjusted by

selecting HI or LO gain mode.

The relative noise and distortion performance of the two gain

modes can be compared in Figure 21 and Figure 27 through

Figure 37. The 48 nV/√Hz noise floor of the LO gain mode is

suited to converters with higher sampling rates or resolutions

(such as 12 bits). Both gain modes can accommodate ADC full-

scale voltages as high as 4 V p-p. Since distortion performance

remains favorable for output voltages as high as 4 V p-p (see

Figure 32), it is possible to lower the output-referred noise even

further by using a resistive attenuator (or transformer) at the

output. The circuit in Figure 71 has an output full-scale range of

2 V p-p, a gain range of –10.5 dB to +37.5 dB, and an output

noise floor of 24 nV/√Hz, making it suitable for some 14-bit

ADC applications.

VOH

VOL

LPF

4V p-p DIFF,

48n V/

187

Ω

2V p-p DIFF,

24n V/

2:1

374

Ω

187

Ω

ADC

AD6644

Figure 71. Adjusting the Noise Floor for 14-Bit ADCs

OVERLOAD

These devices respond gracefully to large signals that overload

its input stage and to normal signals that overload the VGA

when the gain is set unexpectedly high. Each stage is designed

for clean-limited overload waveforms and fast recovery when

gain setting or input amplitude is reduced.

Signals larger than ±275 mV at the LNA input are clipped to

5 V p-p differential prior to the input of the VGA. Figure 44

shows the response to a 1 V p-p input burst. The symmetric

overload waveform is important for applications, such as CW

Doppler ultrasound, where the spectrum of the LNA outputs

during overload is critical. The input stage is also designed to

accommodate signals as high as ±2.5 V without triggering the

slow-settling ESD input protection diodes.

Both stages of the VGA are susceptible to overload. Postamp

limiting is more common and results in the clean-limited

output characteristics found in Figure 45. Under more extreme

conditions, the X-AMP will overload, causing the minor glitches

evident in Figure 46. Recovery is fast in all cases. The graph in

Figure 72 summarizes the combinations of input signal and

gain that lead to the different types of overload.

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