Engineers developing precision measurement and data acquisition signal chains are constantly challenged to combine precision and stability to minimize the overhead of system recalibration. How do the signal chain components influence precision and stability?
Digital Multimeter Example
Digital multimeters (DMMs) measure DC voltages and low frequency AC signals. They are classified according to the number of digits (resolution) that can be reliably reported within a specified accuracy. A 7.5-digit DMM falls into the category that is suitable for high precision applications and metrology reference standards. These can include:
- Calibrators for field instruments
- Lab grade weight scales
- Seismic instruments
- ATE instruments
A basic data acquisition signal chain for a DDM is shown below. It has an input stage, followed by a driver stage for the ADC. An external voltage reference determines the input range for the ADC. The input stage should have high impedance to avoid loading the source being measured. The anti-aliasing filter between the input stage and the driver stage bandlimits the noise. The driver stage performs multiple functions: (a) provides a high impedance load for the filter; (b) attenuation of the signal to fit within the ADC input range; and (c) provides sufficient drive to the ADC to settle the input.
Figure 1. Example DMM data acquisition signal chain.
Measurement Challenges
- For DC measurements, temperature drift can add micro-volts to milli-volts of error to the measurement due to changes in offset voltage and bias current. Selecting components with low temperature coefficients, such as zero-drift amplifiers and oven-controlled buried Zener voltage references, helps to diminish the frequency with which the system may need to be re-calibrated as the temperature varies.
- Both DC and AC measurements are impacted by noise as temperature increases due to the kTR Selecting low noise components can diminish this impact. An additional mitigation is to use an ADC that includes a digital averaging function, which improves the SNR by 3 dB for each increase in the averaging factor by a power of 2.
- Finally, in a sampled data system, a voltage transient appears at the ADC input at the beginning of the sample acquisition phase due to charge redistribution. The voltage transient must settle before the ADC starts conversion; else precision is compromised.
Figure 2. Transient response at ADC input.
An example DMM signal chain that achieves 7.5 digit accuracy is shown below.
In this specific example, the input can range between +/- 10 volts. The ADA4523-1 is a low noise zero-drift op amp used for the input stage and driver stage. Its high common mode input impedance (100GΩ) avoids loading the source. Its noise density (4.2 nV/ÖHz), low offset (VOS < + 5µV max.), low bias current (600 pA max.), and low offset drift (+ 0.02 µV/oC drift max.) support the accuracy and stability over temperature that is needed to achieve 7.5-digit performance.
To complete the signal chain, and as critical as the front end, is the AD4630-24, a 24-bit, 2 MSPS Easy Drive SAR ADC. Like ADI’s other Easy Drive SAR ADCs, the AD4630-24 employs an innovative input architecture to reduce the voltage transients during sampling, which eases the requirements on the driver.
The AD4630-24 reduces the voltage transient by pre-charging the sampling capacitor to the previous sample voltage. This reduces the voltage transients on the ADC inputs, leading to faster settling time and lower settling error. This enables the ADA4523-1 to be used for the driver stage.
The AD4630-24 is a dual channel Easy Drive SAR ADC, while the AD4030-24 is a single channel version (offering improved SNR). Both have excellent linearity (INL: +0.1 ppm typ., +0.9 ppm max.), low zero error (0 µV typ., + 90 µV max.), and low zero error drift (+ 0.007 ppm/oC). As important, both have programmable digital gain and offset adjustments which can be used to compensate for the static non-idealities in the analog front end. They also offer selectable averaging that can be used to reduce noise and improve SNR.
The ADR1001 is an oven-controlled buried Zener voltage reference that has a 0.2 ppm/oC temperature coefficient and extremely low long-term drift (5 ppm, 2000 hours).
Check out the following references to learn more about this DMM solution.
- https://www.analog.com/en/resources/technical-articles/achieve-7-pt-5-digits-accuracy-instrumentation-apps-part1.html
- https://www.analog.com/en/resources/technical-articles/achieve-7-pt-5-digits-accuracy-instrumentation-apps-part2.html
To learn more about Easy Drive circuit design techniques, see the following references.
- Tech Note: Can Analog Devices' Easy Drive ADCs simplify multi-channel sensor signal chain design?
- Video: Analog Devices EasyDrive
︎ ADCs - Blog: LTspice®︎ and EasyDrive︎ — A One-Two-Punch for Design Efficiency
- KWIK Note: SAR ADC Kickback Filter Design
- Tech Note: Can Analog Devices’ Easy Drive ADCs save cost and board space?
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Featured Products |
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AD4630-24: 24-Bit, 2 MSPS Dual Channel Easy Drive SAR ADC AD4030-24: 24-Bit, 2 MSPS Single Channel Easy Drive SAR ADC
The AD4630-24 and AD4030-24 offer the benefit of Easy Drive, highly linear input networks, programmable digital gain and offset compensation, and programmable averaging filter to achieve the ultimate in precision performance. |
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ADA4523-1: 36 V, Low Noise, Zero Drift Op Amp The ADA4523-1 zero-drift design achieves VOS + 5µV max., + 0.02 µV/oC drift max.
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ADR1001: Oven-Controlled, Buried Zener, Precision Voltage Reference The ADR1001 offers the best combination of extreme temperature stability, initial accuracy (0.25%), and low noise (700 nVp-p). |
Explore and compare the full offering of Easy Drive ADCs from the Analog Devices Product Selection Table.