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Digital predistortion boosts data-acquisition performance

08 Oct 2015  | Srudeep Patil, Carmelo Morello

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A data acquisition system (DAS) converts analogue signals to digital for analysis by a digital signal processor. Applications such as imaging, audio, and vibration analysis require a DAS with a high signal-to-noise ratio (SNR) and ultra-low total harmonic distortion (THD).

Developing a wide-dynamic-range DAS does, however, present design and test challenges. The main test challenge is the lack of signal source with a better THD and SNR than the DAS itself. Both the THD and SNR of a signal generator become critical concerns when the DAS is rated for 100 dB SNR and -120 dB THD.

Why improve source distortion?
To characterise the THD of a DAS, you should connect an ideal sinewave with no distortion to the system input. From that, you can measure the THD introduced by DAS non-linearity.

To guarantee that the THD measured at the output derives from DAS non-linearity, you need to use a generator with negligible distortion compared to that of the DAS under test. Most signal generators, however, aren't always good enough to measure the ultra-low-distortion DAS with better than -120dB THD.

DAS and measurement setup
The DAS under test has been designed using low distortion and low-noise devices on the main signal path. We performed experiments using the MAX11905DIFEVKIT, a wide-dynamic-range and ultra-low-distortion DAS, which consists of three three major components. Table 1 highlights the specs of these devices.

MAX44205: a fully differential amplifier, 180MHz gain bandwidth product, 3 nVRMS noise

MAX11905: a fully differential SAR ADC, 20bit, 1.6 Msample/s, low power

MAX6126: an ultra-high-precision, ultra-low-noise series voltage reference

Table: Noise and distortion performance of the signal-chain.

Figure 1 shows the test setup to evaluate the DAS dynamic performance. The low-distortion signal generator is the Audio Precision AP 2722.

Figure 1: This test setup measures dynamic performance of the DAS.

We used the Audio Precision 2722 to generate a fully differential 10kHz sinusoidal signal. We applied it to the MAX44205 driver configured with a gain = 1 V/V. The MAX11905 ADC is used in fully differential mode with VREF = 3 V supplied from the MAX6126Vage reference. The signal analyser and ADC are synchronised with the same clock generator to implement coherent sampling measurement.

DAS Performance using a traditional approach
The results in figure 2 show the dynamic performance of the DAS with the setup configured in figure 1. The ADC is running at a sampling rate of 1.6 Msamples/s.

Figure 2: The plot shows initial FFT and dynamic performance of the DAS system tested on the MAX11905DIFFEVKIT at 1.6 Msamples/s.

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