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ADG5207 Datasheet(PDF) 3 Page - Analog Devices |
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ADG5207 Datasheet(HTML) 3 Page - Analog Devices |
3 / 13 page Circuit Note CN-0385 Rev. 0 | Page 3 of 13 The AD8475 funnel amplifier provides high precision attenuation (0.4×), accurate common-mode level shifting, and single-ended to differential conversion. Its low output noise spectral density (10 nV/√Hz) and fast settling time (50 ns to 0.001% for a 2 V output step) make it well suited to drive the AD4003. The AD4003 is a fully-differential, 2 MSPS, 18-bit precision SAR ADC that features a typical signal-to-noise ratio (SNR) of 98 dB when using a 4.096 V reference. The AD4003 is also low power, and only consumes approximately 17 mW at full throughput. Its power consumption scales with throughput, and can operate at lower sample rates to cut its power use (for example, 0.17 mW at 100 kSPS). System DC Accuracy Errors Figure 2 shows the ideal transfer function of the data acquisition system. Figure 2. ADC Ideal Transfer Function Each of the components in the data acquisition signal chain adds its own offset error and gain error that cause the real transfer function of the system to deviate from the ideal transfer function shown in Figure 2. The cumulative effects of these errors can be measured at a system level by comparing known dc inputs near zero and full scale at the input to the ADG5207 (RC filter if it is present) and the resulting output codes from the AD4003 to obtain a system calibration factor. Offset Error Measurement For ideal bipolar, differential ADCs, a 0 V differential input results in an output code of 0. Real ADCs typically exhibit some offset error (εb), which is defined as the deviation between the ideal output code and the measured output code for a 0 V input. The offset error for the data acquisition system can be found by grounding its input and observing the resulting output code. This error varies between each of the gain settings of the AD8251 and between each of the channels of the ADG5207. Offset error is therefore measured for each of the channels in all four gain configurations. Because the system monitors multiple channels, it is also important to quantify the amount by which the offset error deviates between channels. Offset error match (Δεb,MAX) is a measure of the max- imum deviation between the offset error of each of the channels and the average offset error of all of the channels. Offset error match is calculated using the following equation: ) 7 ..., ,1 , 0 | ) 8 (max( 7 0 , , , = ε − ε = ε ∆ ∑ = i j j b i b MAX b where εb,i and εb,j are the offset errors for the i and j channels, respectively. This offset error match can be found for each of the gain configurations. Note that offset error can be expressed either in codes or volts. Gain Error Measurement Error in the gain of the system also contributes to overall system inaccuracy. The ideal transfer function of the AD4003 is shown in Figure 2, where the −217 and 217 − 1 output codes correspond to a negative full-scale input voltage (−FS) and a positive full-scale input voltage (+FS), respectively; however, the combination of offset error (εb) and gain error (εm) results in a deviation from this relationship. Gain error can be expressed as a percentage error between the actual system gain and the ideal system gain. The more common expression is in percent full-scale error (%FS), which is a measure of the error between the ideal and actual input voltages that produces the 217 − 1 code. The ideal full-scale input voltage (VFS,IDEAL) is a function of the resolution of the ADC (18-bits for the AD4003) and the accuracy of the reference voltage (VREF). Errors in the voltage reference translate to gain errors in the ADC. To decouple reference errors from ADC gain error, VREF is measured using a precision multimeter. The ideal full-scale input voltage can then be calculated using MEAS REF MEAS REF IDEAL FS V V V , 17 , 18 , 2 2 2 = × = The actual system gain can be found by calculating the slope of the linear regression of a group of several input voltages (mLR) and the resulting output codes: YREAL = mLR × VIN The real full-scale input voltage (VFS,REAL) can then be calculated using LR LR REAL REAL FS m m Y V 17 , 2 = = The gain error (expressed in %FS error) can then be calculated using % 100 , , , × − = ε IDEAL FS REAL FS IDEAL FS m V V V The gain error of the system varies with the gain of the AD8251, but is channel independent. Therefore, gain error is measured for each of the four gain configurations, but only using one of the ADG5207 channels in this system. 100...000 100...001 100...010 011...101 011...110 011...111 ANALOG INPUT +FSR – 1.5 LSB +FSR – 1 LSB –FSR + 1 LSB –FSR –FSR + 0.5 LSB |
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