Figure 1. Test configuration circuite.
Several steps were taken to minimize the effects of extraneous noise on measurement data. Shielded cabling was used on all inputs and outputs from (or to) the DS1802 and NE5532 Op Amps. All ground signals were tied to a common point and power supplies were capacitively coupled to ground using 10 µF capacitors. Additionally, both the DS1802 and NE5532 Op Amps used separate power supplies.
Once assembled, this circuit was mounted inside a metal testbox to provide additional protection against coupling of random noise from external sources such as computer monitors, lights, etc. BNC connectors were used for inputs and outputs to test equipment.
Graphical data is presented in Figures 2 and 3 for a sample set consisting of five units of the DS1802. Figure 2 presents actual data for the five devices tested. Figure 3 further summarizes this data by presentation of the calculated average. The absolute error data taken in these tests indicate the DS1802 performs well within specified data sheet limits of ± 0.5 dB.
Figure 2. DS1802 Absolute error.
Figure 3. DS1802 Average absolute error.
Figure 4 provides actual data taken on the units, while Figure 5 shows a summary of the average data. Average data for inter-channel matching indicates that this device characteristic falls within a ± 0.05 dB window; which is well within the data sheet specification of ± 0.25 dB.
Figure 4. Inter-channel matching.
Figure 5. Inter-channel matching summary.
Figure 6. %THD+N vs. Frequency 80kHz bandlimited.
Figure 7. %THD+N vs. Frequency 30kHz bandlimited.
Figure 8. %THD+N vs. Frequency 22kHz bandlimited.
By setting the wiper position of the potentiometer at some level other than position-0 allows the potential user of the DS1802 to view distortion of the part as the input signal covers some portion of the resistor array. A position setting of 0 primarily allows the examination of the input signal source. The position-6 setting used in these tests provides a worst case selection for monitoring %THD+N.
The DS1802 is specified for a %THD+N of 0.002% at 1kHz. This specification is derived using the 80kHz bandlimited input signal at 1Vrms, and a position-6 potentiometer setting. Across the 20Hz to 20kHz frequency range %THD+ N is less than 0.02% maximum with the 80kHz bandlimited signal. Additionally, %THD+N across the 20Hz to 20kHz frequency range is significantly improved using bandlimited signals in conjunction with the A-weighting filter.
The A-weighting filter is a specific noise weighting filter ( ANSI S1.4, IEC Recommendation 179) used to produce noise measurements which correspond well with human observations over the audio frequency range.1 More detailed information concerning this filter can be found in the ANSI specification stated above or in the Audio Measurement Handbook published by Audio Precision, Incorporated.
Figure 9. SMPTE Intermodulation distortion vs. amplitude.
Figure 10. CCIF Intermodulation distortion vs. amplitude.
The SMPTE (Society of Motion Picture and Television Engineers) standard specifies a two-sinewave test signal consisting of a low-frequency, high-amplitude tone linearly combined with a high-frequency sinewave at 1/4 the amplitude of the low-frequency tone. The SMPTE specification calls for 60Hz and 7kHz as the two sinewaves. When a non-linear device is subjected to a two-tone test signal, intermodulation products appear as sidebands around the high-frequency tone. The percentage intermodulation distortion is defined as the percentage of amplitude modulation, represented by the second and third order pair of sidebands, of the high-frequency signal. Second order sidebands around the high frequency tone are spaced at a frequency equal to the low-frequency tone (FH ± FL). Third order sidebands are spaced at twice the low-frequency tone or FH ± 2FL. (FH and FL correspond to the highfrequency and low-frequency tones, respectively.2)
CCIF intermodulation distortion testing consists of using two equal-amplitude, high-frequency signals spaced closely in frequency. Signals used for test data presented include 13kHz and 14kHz. The audio analyzer measures only the amplitude of the difference tone or low-frequency product resulting from the two high-frequency test signals. Here, the percentage intermodulation distortion is defined as the percentage of amplitude modulation which the low order or difference signal (FH - FL) represents on the high-frequency signals. Additional information on SMPTE and CCIF can be found in the Audio Measurement Handbook and System One Audio Analyzer User's Manual published by Audio Precision, Incorporated.3
Figure 11. Cross-talk configuration potentiometer 1 open.
Figure 12. Cross-talk configuration potentiometer 1 grounded.
Potentiometer 1 and its wiper (primarily the wiper) are used as the comparison point for signal feedthrough. The high and low terminals, H1 and L1, are either left open circuited, as shown in Figure 11 or grounded as shown in Figure 12. The wiper of potentiometer 1 is connected to the audio analyzer's input via channel-B.
A comparison between the channel-A input and channel-B input of the audio analyzer is then made to determine the level of cross-talk. Graphical data of both the configurations of Figures 11 and 12 are presented in Figure 13.
Figure 13. Cross-talk data.