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ADS1120-Q1 Datasheet(PDF) 44 Page - Texas Instruments |
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ADS1120-Q1 Datasheet(HTML) 44 Page - Texas Instruments |
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44 / 67 page  ADS1120-Q1 SBAS683A – AUGUST 2014 – REVISED OCTOBER 2014 www.ti.com Application Information (continued) Many sensor signals are inherently bandlimited; for example, the output of a thermocouple has a limited rate of change. In this case the sensor signal does not alias back into the pass-band when using a ΔΣ ADC. However, any noise pick-up along the sensor wiring or the application circuitry can potentially alias into the pass-band. Power line-cycle frequency and harmonics are one common noise source. External noise can also be generated from electromagnetic interference (EMI) or radio frequency interference (RFI) sources, such as nearby motors and cellular phones. Another noise source typically exists on the printed circuit board (PCB) itself in the form of clocks and other digital signals. Analog input filtering helps remove unwanted signals from affecting the measurement result. A first-order resistor-capacitor (RC) filter is (in most cases) sufficient to either totally eliminate aliasing, or to reduce the effect of aliasing to a level within the noise floor of the sensor. Ideally, any signal beyond f(MOD) / 2 is attenuated to a level below the noise floor of the ADC. The digital filter of the ADS1120-Q1 attenuates signals to a certain degree, as illustrated in the filter response plots in the Digital Filter section. In addition, noise components are usually smaller in magnitude than the actual sensor signal. Therefore, using a first-order RC filter with a cutoff frequency set at the output data rate or 10x higher is generally a good starting point for a system design. Internal to the device, prior to the PGA inputs, is an EMI filter as shown in Figure 39. The cutoff frequency of this filter is approximately 31.8 MHz, which helps reject high-frequency interferences. 9.1.3 External Reference and Ratiometric Measurements The full-scale range of the ADS1120-Q1 is defined by the reference voltage and the PGA gain (FSR = ±Vref / Gain). An external reference can be used instead of the integrated 2.048-V reference to adapt the FSR to the specific system needs. An external reference must be used if VIN > 2.048 V. For example, an external 5-V reference and an AVDD = 5 V are required in order to measure a single-ended signal that can swing between 0 V and 5 V. The reference inputs of the device also allow the implementation of ratiometric measurements. In a ratiometric measurement the same excitation source that is used to excite the sensor is also used to establish the reference for the ADC. As an example, a simple form of a ratiometric measurement uses the same current source to excite both the resistive sensor element (such as an RTD) and another resistive reference element that is in series with the element being measured. The voltage that develops across the reference element is used as the reference source for the ADC. Because current noise and drift are common to both the sensor measurement and the reference, these components cancel out in the ADC transfer function. The output code is only a ratio of the sensor element and the value of the reference resistor. The value of the excitation current source itself is not part of the ADC transfer function. 9.1.4 Establishing a Proper Common-Mode Input Voltage The ADS1120-Q1 can be used to measure various types of input signal configurations: single-ended, pseudo- differential, and fully-differential signals (which can be either unipolar or bipolar). However, configuring the device properly for the respective signal type is important. Signals where the negative analog input is fixed and referenced to analog ground (V(AINN) = 0 V) are commonly called single-ended signals. The common-mode voltage of a single-ended signal consequently varies between 0 V and VIN / 2. If the PGA is disabled and bypassed, the common-mode input voltage of the ADS1120-Q1 can be as low as 100 mV below AVSS and as large as 100 mV above AVDD. Therefore, the PGA_BYPASS bit must be set in order to measure single-ended signals when a unipolar analog supply is used (AVSS = 0 V). Gains of 1, 2, and 4 are still possible in this configuration. Measuring a 0-mA to 20-mA or 4-mA to 20-mA signal across a load resistor of 100 Ω referenced to GND is a typical example. The ADS1120-Q1 can directly measure the signal across the load resistor using a unipolar supply, the internal 2.048-V reference, and gain = 1 when the PGA is bypassed. If gains larger than 4 are needed to measure a single-ended signal, the PGA must be enabled. In this case, a bipolar supply is required for the ADS1120-Q1 to meet the common-mode voltage requirement of the PGA. Signals where the negative analog input (AINN) is fixed at a voltage other the 0 V are referred to as pseudo- differential signals. The common-mode voltage of a pseudo-differential signal varies between V(AINN) and V(AINN) + VIN / 2. 44 Submit Documentation Feedback Copyright © 2014, Texas Instruments Incorporated Product Folder Links: ADS1120-Q1 |
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