Digital predistortion improves data-acquisition performance

September 29, 2015 // By Srudeep Patil and Carmelo Morello
Srudeep Patil and Carmelo Morello of Maxim Integrated explains how digital predistortion can improve data-acquisition performance.

A data acquisition system (DAS) converts analog 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 THD (total harmonic distortion).

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 characterize 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 nonlinearity.

To guarantee that the THD measured at the output derives from DAS nonlinearity, 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, 180 MHz gain bandwidth product, 3 nVRMS noise
  • MAX11905: a fully differential SAR ADC, 20-bit, 1.6 Msample/s, low power
  • MAX6126: an ultra-high-precision, ultra-low-noise series voltage reference

Table 1. 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 10 kHz sinusoidal signal.

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