Data acquisition
the accuracy and integrity of the measured signal. Finally, high resolution ADCs are used to convert the analogue signal into a digital format for further processing or analysis. These ADCs have high sampling rates and excellent resolution, allowing for precise and detailed digitisation of the analogue signal. All these components are handpicked to achieve the desired performance of the reference platform.
Delving into the specifics, the data acquisition board within the reference platform showcases a discrete programmable gain instrumentation amplifier (PGIA) composed of several components, including:
ADA4627-1: high speed, low noise, low bias current, JFET operational amplifier
LT5400: precision quad matched resistor network
ADG1209: low capacitance, 4-channel, ±15V/+12V iCMOS multiplexer
Internal fully differential amplifier (FDA) ADC driver of the ADAQ4003
The PGIA at the front end offers high input impedance that allows direct interface with a variety of sensors. A programmable gain is often needed to adapt the circuit to different input signal amplitudes - unipolar or bipolar and single-ended or differential with varying common-mode voltages. The PGIA works with the ADAQ4003, an 18-bit, 2MSPS, µModule data acquisition solution. Figure 4 illustrates the entire signal chain of this reference platform. In order to verify the static performance of the reference platform, the integral non-linearity (INL) and differential non-linearity (DNL) were measured, respectively. Figures 5 and 6 illustrate the DNL and INL errors vs. code across various gains. The DNL errors have typical deviations of ±0.6 LSB, denoting a monotonic transfer function with no missing codes. Meanwhile, the INL errors have typical deviations of ±2.097 LSB with a visible S-shape, indicating the strong predominance of odd-ordered harmonics. These graphs depict that sufficient linearity is obtained from the entire signal chain. Employing precision amplifiers, signal conditioning techniques, and high resolution ADCs in the signal chain minimises signal distortion, offsets, and non-linearities, resulting in highly accurate measurements. The galvanic isolation techniques discussed earlier further reduce common-mode voltage variations and eliminate ground loop effects, ensuring an accurate representation of the measured signal.
MINIMISING NOISE AND INTERFERENCE Noise and interference are also common challenges in data acquisition, which either emanate from components or external sources. An isolated precision signal chain addresses these issues by employing
60 Figure 5. DNL vs. code for various gains, VREF = 5V.
Figure 6. INL vs. code for various gains, VREF = 5V.
robust isolation barriers, shielding, grounding, and filtering techniques. Noise reduction techniques are incorporated in the µModule ADAQ4003 itself, enabling a high fidelity signal capture. In particular, a single-pole, low-pass RC filter is placed between the ADC driver output and the ADC inputs inside the µModule device - which serves the following purposes: (1) eliminates high frequency noise, (2) reduces the charge kickbacks from the input of the internal SAR ADC, and (3) maximises settling time and input signal bandwidth. The layout of the µModule device also ensures that the analogue and digital paths are separated to avoid crossover of these signals and ease the radiating noise. Although there are many dynamic parameters correlating to the performance of a certain data acquisition system, only three will be discussed in this article.
Dynamic range is defined as the range between the noise floor of a device and its specified maximum output level, which is essential in determining the smallest voltage increment that is not affected by the noise. This parameter is tested using a 5V reference with inputs shorted to ground, at an output data rate of 2MSPS. Figure 7 shows the dynamic range across various gains, with a typical value of 93dB (at the highest gain setting) and 100dB (at the lowest gain setting). Increasing the oversampling ratio to a factor of 1024× further improves the measurement, reaching up to a maximum of 123dB and 130dB, respectively.
September 2025 Instrumentation Monthly
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