D4a - 2 channel 24-bit ADC

Description

The D4a module contains 2 individual 24-bit ADCs (Analog to Digital Converters) with an input range of ±3.5V. It is designed as alternative to Keithley DMMs, and offers similar or better performance in terms of noise at low magnitudes near 0V and 50 Hz rejection and comparable temperature drift. For details, see specifications.

As can be seen in the diagram below, the ADCs are two separate units: they operate independently. This means they are able to run at different sample rates and can be triggered separately. So one D4a module could replace two Keithley DMMs.

D4a block diagram

The D4a is based on a now-obsoltete D4 module. If you still have a D4 module you can find its specification on the original D4 webpage.

Filter Settings

The ADCs inside the module are equipped with two types of built-in digital filters: a Sinc3 filter and a Sinc5+Sinc1 filter. These filters determine to a large extent the performance of the module. By changing the filter setting, the user sets the Data Rate, Resolution, Bandwidth and the 50/60 Hz rejection. The default filter type is Sinc3 which is preferred for a single-input with a constant sampler configuration (non multiplexed sampling). Combined with filter setting of 20 as default it provides a very high rejection of 50Hz / 60Hz power line fluctuations. Other filter settings also provide excellent 50Hz rejection are 19, 17, and 15, which provide higher bandwidth and data rates at the expense of noise.

For a complete list of filter types and settings, see the D4_Filter_50Hz_60Hz_AnlgBuffersDisabled.xlsx file below.

  • D4_Filter_50Hz_60Hz_AnlgBuffersDisabled.xlsx (13 ko)

    • Specifications

    Parameter Value Units Conditions
    Range ±3.5 V
    Resolution 0.4 uV At default conditions and ±3.5V scale. The resolution filter dependent. See `D4a_Filter_50Hz_60Hz.xlsx` for the full list of settings
    Effective Data Rate 1.67 SPS Default (slowest). This rate is determined by the defaut settings. Faster data rates are achieveable by a change of filter type and internal settling time with a possible impact on other parameters (noise, resolution, bandwidth, 50 / 60 Hz rejection).
    1K SPS Fastest
    Input Resistance 10 MΩ
    Temperature Coefficient ±2.5 ppm/°C
    INL -150 to +100 uV Across full ±3.5V range
    Noise 3 uV Noise floor
    57 uV / V Proportional to input magnitude within a ±1V range
    33 uV / V Additional input magnitude beyond ±1V
    50 Hz rejection 103 dB At 1.67 SPS
    60 Hz rejection 107 dB At 1.67 SPS