Data acquisition
Figure 5. LTC6373 driving ADAQ4003 (gain = 0.454, 2 MSPS).
the necessary (low equivalent series resistance (ESR) and low equivalent series inductance (ESL)) decoupling ceramic capacitors for the reference (REF) and power supply (VS+, VS−, VDD, and VIO) pins. These capacitors provide a low impedance path to ground at high frequencies to handle transient currents. There are no external decoupling capacitors
required and, in the absence of these capacitors, there is no known performance impact or any EMI issue. This performance impact was verified on the ADAQ4003 evaluation board by removing the external decoupling capacitors on the output of reference and LDO regulators that generate the on-board rails (REF, VS+, VS−, VDD, and VIO). Figure 4 shows that any spurs are buried well below −120 dB in the noise floor regardless of whether the external decoupling capacitors are used or removed. The ADAQ4003’s small form factor enables the high channel density PCB layout while mitigating thermal challenges. However, the placement of individual components and routing of various signals on the PCB is crucial. The symmetrical routing of input and output signals while keeping the power supply circuitry away from the analog signal path on a separate power layer with as large of a trace as possible is especially crucial to provide low impedance paths and reduce the effects of glitches on the power supply lines and avoid the EMI type issue.
Driving the ADAQ4003 Using A high impeDAnce pgiA As previously discussed, high input impedance front ends are typically required to directly connect with various types of sensors. The majority of instrumentation and programmable gain instrumentation amplifiers (PGIAs) have single-ended outputs, which cannot directly drive the fully differential data acquisition signal chain. However, the LTC6373 PGIA offers fully differential outputs, low noise, low distortion, and high bandwidth, which can directly drive the ADAQ4003 without sacrificing precision performance, making it suitable to many signal chain applications. The LTC6373 is dc-coupled on the input and the output with programmable gain settings (using the A2, A1, and A0 pins). In Figure 5, the LTC6373 is used in a differential
input to differential output configuration and dual supplies of ±15 V. The LTC6373 can also be used in a single-ended input to differential output
46 Figure 8. INL/DNL performance, with the LTC6373 (gain = 1) driving the ADAQ4003 (gain = 0.454). May 2021 Instrumentation Monthly
configuration if required. The LTC6373 directly drives the ADAQ4003 with its gain set as 0.454.
The VOCM pin of the LTC6373 is connected to ground and its outputs swing between −5.5 V and +5.5 V (opposite in phase). The FDA of the ADAQ4003 level shifts the outputs of the LTC6373 to match the desired input common mode of the ADAQ4003 and provides the signal amplitude necessary to utilize the maximum 2 ×
VREF peak-to-peak differential signal range of the ADC inside the ADAQ4003 µModule device. Figure 6 and Figure 7 show the SNR and THD performance using various gain settings of the LTC6373, while Figure 8 shows the INL/DNL performance of ±0.65 LSB/±0.25 LSB for the circuit configuration shown in Figure 5.
ADAQ4003 μmoDUle ApplicAtion Use cAse: Ate This section will focus on how the ADAQ4003 makes a great fit for source measurement units (SMUs) and device power supplies (DPSs) for ATE. These modular instruments are used to test a wide variety of chip types for the rapidly growing smartphone, 5G, automotive, and IoT markets.
Figure 6. SNR vs. the LTC6373 gain setting, with the LTC6373 driving the ADAQ4003 (gain = 0.454, 2 MSPS).
Figure 7. THD vs. the LTC6373 gain setting, with the LTC6373 driving the ADAQ4003 (gain = 0.454, 2 MSPS).
These precision instruments have a sink/source capability, which requires a control loop for each channel that takes care of the programmed voltage and current regulation, and they demand high accuracy (especially fine linearity), speed, wide dynamic range (to measure µA/µV signal levels), monotonicity, and a small form factor to
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