–8–

SMP04

REV. D

Optimizing Dynamic Performance of the SMP04

Various operating parameters such as input voltage amplitude,

sampling pulsewidth and, as mentioned before, supply bypass-

ing and grounding all have an effect on the signal-to-noise ratio.

Table I shows the SNR versus input level for the SMP04.

Distortion of the SMP04 is reduced by increasing the supply

voltage. This has the effect of increasing the positive slew rate.

Table II shows data taken at 12.3 kHz sample rate and 2 kHz

input frequency. Total harmonic distortion is dominated by the

second and third harmonics.

FREQUENCY DOMAIN PERFORMANCE

The SMP04 has been characterized in the frequency domain for

those applications that require capture of dynamic signals. See

Figure 16a for typical 86.1 kHz sample rate and an 8 kHz input

signal. Typically, the SMP04 can sample at rates up to 85 kHz.

In addition to the maximum sample rate, a minimum sample

pulsewidth will also be acceptable for a given design. Our testing

shows a drop in performance as the sample pulsewidth becomes

less than 4

µs.

10 dB/DIV

RANGE 15.0 dBm

6.0 dBm

START 1 000.0 Hz

STOP 100 000.0 Hz

a.

10 dB/DIV

RANGE 15.0 dBm

6.3 dBm

START 1 000.0 Hz

STOP 100 000.0 Hz

b.

Figure 16. Spectral Response at a Sampling Frequency of

86 kHz. Photo (a) Shows a 20 kHz Carrier Frequency and

Photo (b) Shows an 8 kHz Frequency.

Table III shows the effect of sampling pulsewidth on the SNR of

the SMP04. The recommended operating pulsewidth should be

a minimum of 5

µs to achieve a good balance between acqui-

sition time and SNR for the 1.4 V p-p signal shown. For larger

swings the pulsewidth will need to be larger to account for

the time required for the signal to slew the additional voltage.

This could be used as a method of measuring acquisition

time indirectly.

Input

Voltage

SNR

(V p-p)

(dB)

1

–61

2

–53

3

–50

4

–47

5

–45

6

–44

Table II. SNR vs. Supply Voltage

Supply

Voltage

2nd

3rd

(V)

(dB)

(dB)

10

–49

–62

12

–55

–71

14

–60

–80

15

–62

<–80

16

–63

<–83

17

–65

<–85

Table III. SNR vs. Sample Pulsewidth

Sample

Pulsewidth

SNR

( s)

(dB)

1

–37

2

–44

3

–50

4

–54

5

–54.9

6

–55

7

–55.3