Test & measurement T
WHY DOES VOLTAGE REFERENCE NOISE MATTER?
here is increasing demand for ultrahigh precision measurements that can achieve greater than 24-bit resolution in industries from aerospace and defence and gas exploration to pharmaceutical and medical device manufacturers. For example, the pharmaceutical
industry uses high precision lab balances that offer 0.0001mg resolution over a 2.1g full-scale range that would require an analogue-to- digital converter (ADC) with greater than 24-bit resolution. Calibration and testing of these high precision system challenges the instrumentation industry to offer test equipment that can achieve greater than 25-bit resolution with at least 7.5 digit measurement precision.
To achieve this high resolution, a signal chain with exceptionally low noise is required. Figure 1 shows the relationship of noise vs. effective number of bits (ENOB) and signal-to-noise ratio (SNR). Note, noise is calculated based on voltage reference (VREF) equal to 5V and ADC input set to full-scale range. To provide perspective, to achieve 25-bit resolution, or 152dB dynamic range, the maximum allowable system noise is 0.2437µV rms. The voltage reference sets the limit to the input
a low noise specification needs some help to attenuate its noise. Adding external circuitry such as a filter can help attenuate noise to achieve the desired ADC dynamic range.
Figure 2. Relationship of ADC VIN with rms system noise. VREF set to LTC6655-5.
true because VIN is set to 0V, resulting in the ratio VIN/VREF being equal to 0V.
To investigate the effects of voltage reference noise on overall system noise, Figure 2 shows the relationship between total system noise (rms) with ADC input dc source voltage. For this test, we used the AD7177-2 32-bit ADC with the VREF input connected to the LTC6655-5 (5V) and the ADC input connected to a low noise dc source. The ADC output data rate was set to 10kSPS. Note, throughout the ADC input voltage range, the ADC noise remains constant (35nV/√Hz) while the ADC dc input source noise rises (≤6nV/√Hz) but remains low in comparison to the voltage reference noise (96nV/√Hz). As shown in Figure 2, the total noise is proportional to the ADC dc input voltage. This is because as VIN increases, the ratio VIN/VREF increases and so the VREF noise dominates the overall system noise when the ADC is at full-scale input. The individual noise of each component in the signal chain adds together in root sum square (RSS) fashion and gives rise to the shape of the curve in Figure 2.
Figure 1. Noise vs. ENOB and SNR.
analogue signal that the ADC can resolve. Equation 1 is the ideal transfer function of an ADC where the output code - in decimal form - is computed by the analogue input signal VIN, voltage reference VREF, and number of ADC bits N.
Typically, the resolution stated in the ADC data sheet is based on an input shorted technique where the ADC input is connected to the GND or the ADC differential inputs are connected to a common source. The ADC input shorted technique helps to characterise the absolute limit of the ADC resolution by omitting the ADC input source noise and eliminating the effect of VREF noise. This is
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To achieve a high measurement resolution of 25 bits or beyond, even the best standalone voltage reference available in the market with
The remainder of this article explains various types of low-pass filters and how they can be applied to attenuate voltage reference noise. Filter design techniques and filter trade-offs will be discussed. Two types of low-pass filters that will be discussed in the context of attenuated voltage reference noise are simple passive RC low-pass filters (LPFs) and active-based signal flow graph (SFG) low-pass filters. System evaluation results using a sigma-delta (∑-Δ) ADC will be presented in the circuit performance section.
By Anshul Shah, applications engineer, Analog Devices
Figure 3. Low-pass filter between series voltage reference and ADC.
NOISE REDUCTION USING A PASSIVE LOW-PASS FILTER
Figure 3 shows the voltage reference driving an ADC via a low-pass filter implemented with an external reservoir capacitor, C1, the equivalent series resistance (ESR) of the reservoir capacitor and the output impedance of voltage reference operational amplifier (op amp). The passive RC LPF cutoff frequency is determined by
which states that bandwidth is inversely proportional to resistance R and capacitance C. Reservoir capacitor C1 also works as local energy storage to compensate for voltage spikes caused when ADC voltage reference circuitry demands sudden change in load current. Figure
Figure 4. AD7177-2 and AD7980 simulated dynamic reference current response. May 2025 Instrumentation Monthly
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