SMP04

–9–

REV. D

Sample-Mode Distortion Characteristics

Although designed as a sample-and-hold, the SMP04 may be

used as a straight buffer amplifier by configuring it in a continu-

ous sample mode. This is done by connecting the

S/H control

pin to a logic LOW. Its buffer bandwidth is primarily limited by

the distortion content as the signal frequency increases. Figure

17 shows the distortion characteristics of the SMP04 versus

frequency. It maintains less than 1% total harmonic distortion

over a voiceband of 8 kHz. Output spot noise voltage measures

4 nV/

√Hz at f = 1 kHz.

FREQUENCY – Hz

10

20

100k

1

100

1k

10k

0.1

0.010

0.001

0.0005

200k

Figure 17. THD+N vs. Frequency

Sampled Data Dynamic Performance

In continuous sampled data applications such as voice digitiza-

tion or communication circuits, it is important to analyze the

spectral response of a sample-and-hold. Figures 16a and 16b

show the SMP04 sampling at a frequency of 86 kHz with a

1.4 V p-p pure sine wave input of 20 kHz and 8 kHz respec-

tively. The photos include the sampling carrier frequency as

well as its multiplying frequencies. In the case of the 20 kHz

carrier frequency, the second harmonic measures 41 dB down

from the fundamental, because the second is dominant, the

signal-to-noise ratio is –40.9 dB. The 8 kHz case produces an

improved S/N performance of –48 dB.

In the V.32 and V.33 modem environment, where a 1.8 kHz

carrier signal frequency is applied to the SMP04, Figure 18

compares the spectral responses of the SMP04 under three

different sampling frequencies of 14.4 kHz, 9.6 kHz and

7.2 kHz. The signal-to-noise ratios measure 58.2 dB, 59.3 dB

and 60 dB respectively.

Figure 19 depicts SMP04’s spectral response operating with

voice frequency of 3 kHz sampling at a 15.7 kHz rate. Under

this condition, the signal-to-noise measures 53 dB.

10 dB/DIV

RANGE 15.0 dBm

5.9 dBm

START 1 000.0 Hz

STOP 20 000.0 Hz

Figure 19. SMP04 Spectral Response with an Input Carrier

Frequency of 3 kHz and the Sampling Frequency of 15.7 kHz

Sampled Data Dynamic Performance

In continuous sampled data applications such as voice digitiza-

tion or communication circuits, it is important to analyze the

spectral response of a sample-and-hold. Figures 16a and 16b

show the SMP04 sampling at a frequency of 86 kHz with a

1.4 V p-p pure sine wave input of 20 kHz and 8 kHz respec-

tively. The photos include the sampling carrier frequency as well

as its multiplying frequencies. In the case of the 20 kHz carrier

frequency, the second harmonic measures 41 dB down from the

fundamental, because the second is dominant, the signal-to-

noise ratio is –40.9 dB. The 8 kHz case produces an improved

S/N performance of –48 dB.

In the V.32 and V.33 modem environment, where a 1.8 kHz

carrier signal frequency is applied to the SMP04, Figure 18

compares the spectral responses of the SMP04 under three

different sampling frequencies of 14.4 kHz, 9.6 kHz and

7.2 kHz. The signal-to-noise ratios measure 58.2 dB, 59.3 dB

and 60 dB respectively.

Figure 18. SMP04 Spectral Response with a 1.8 kHz Carrier Frequency. (a) Shows the Sampling Frequency at 14.4 kHz;

it Exhibits a S/N Ratio of 58.2 dB. (b) Shows a 59.3 dB S/N at a Sampling Frequency of 8.6 kHz. (c) Shows a 60 dB S/N at

7.2 kHz.

10 dB/DIV

RANGE 15.0 dBm

5.9 dBm

CENTER 10 500.0 Hz

SPAN 19 000.0 Hz

a.

10 dB/DIV

RANGE 15.0 dBm

5.2 dBm

START 1 000.0 Hz

STOP 12 000.0 Hz

c.

10 dB/DIV

RANGE 15.0 dBm

5.7 dBm

START 1 000.0 Hz

STOP 12 000.0 Hz

b.