Input | ADC Input | Digital Output |
---|---|---|
VinMin = –10V | AIN-xP = –10V, AIN-xGND = 0V | –3276810, 8000H |
VinMax = 10V | AIN-xP = 10V AIN-xGND = 0V | 3276710, 7FFFH |
Power Supplies | |
---|---|
AVDD | DVDD |
5V | 5V |
This cookbook design describes how to select filter component values and how to minimize the gain error and drift introduced by this filter on a fully-integrated analog front end (AFE) SAR ADC. The design uses the input impedance drift at the full scale range of ±10V of the ADS8588S. This external RC filter minimizes external noise and provides protection from electrical overstress. Minimizing gain error and drift are important to end equipment such as: Multi-Function Relays, AC Analog Input Modules, and Terminal Units. This design describes two correction methods, a no-calibration correction factor and a 2-point calibration. Implementing calibration can minimize both the gain error introduced by the external resistor and the internal device gain error to negligible levels.
Specification | Calculated | Measured |
---|---|---|
Introduced Gain Error (25°C) | 0.9901% | 0.9894% |
Introduced Gain Error (125°C) | 0.995% | –1.1388% |
Introduced Gain Error Drift | 0.49ppm/°C | –0.8031ppm/°C |
External anti-aliasing RC filters reduce noise and protect from electrical overstress; if a large resistor value is used, then this further limits the input current. A large external resistive value also provides a low cutoff frequency, which is desired for relay protection applications as the input frequencies are usually 50 or 60Hz. Furthermore, a balanced RC filter configuration is required for better common-mode noise rejection; matching external resistors are present on both the negative and positive input paths. To minimize the introduced drift error, the external resistors needs to be low drift; 25ppm/°C resistors.
REXT = 10kΩ
Nearest standard capacitor value available, CEXT = 24nF
This section demonstrates how to calculate the introduced gain error drift. The additional drift from the external filter resistor is small compared to the internal device drift.
The maximum gain error temperature drift of the ADS8588S is ±14ppm/°C, which is orders of magnitude larger than the calculated drift error introduced, making the introduced error negligible. The minimal drift error introduced by the external resistors has greatly to do with the low drift coefficient of the input impedance (±25ppm/⁰C).
To measure the introduced gain error drift, two test signals are sampled and applied at 0.5V from the full scale input range within the linear range of the ADC. The signals are applied and sampled with and without the external RC filter present. These measurements are performed at both temperatures, 25°C and 125°C. The percent gain errors are solved for by finding the percent error of the ideal slope and the measured slope for each of the four distinctive test conditions, resulting in four distinct percent gain error measurements. The drift (ppm/°C) with and without the RC present is then calculated by converting the percent gain errors to decimal format then following step 5 shown above. The introduced gain error drift is then solved for by subtracting the drift of the RC and no RC present.
An uncalibrated correction targets to solve the input voltage before any losses occur due to the RC filter by working backwards from the ADC measured samples using a voltage divider.
Vin | Measured Code | Equivalent Measured Input |
---|---|---|
9.5V | 30841 | 9.412 |
Using a voltage correction can be beneficial, but not the most comprehensive. The correction factor can have a worst-case error of 0.2456% at room temperature due to change in internal impedance.
Room Temperature (25°C) Measurements | ||||
---|---|---|---|---|
Vin | Code | Reading | Correction | Error % |
9.5 | 30841 | 9.412 | 9.506120 | 0.0644 |
8.5 | 27594 | 8.421 | 8.505210 | 0.0613 |
5 | 16232 | 4.954 | 5.003540 | 0.0708 |
0 | 1 | 0 | 0.000000 | – |
–5 | –16230 | –4.953 | –5.002530 | 0.0506 |
–8.5 | –27593 | –8.421 | –8.505210 | 0.0613 |
–9.5 | –30839 | –9.411 | –9.505110 | 0.0538 |
A two point calibration applies and samples two test signals at 0.5V from the full scale input range within the linear range of the ADC. These sample measurements are then used to calculate the slope and offset of the linear transfer function. Calibration eliminates both the gain error introduced by the external resistor and the internal device gain error.
Vmin | Measured Code |
---|---|
–9.5V | –30839 |
Vmax | Measured Code |
---|---|
9.5V | 30841 |
Calibration Coefficients
m = 3246.3158; b = 1.0001
At room temperature without calibration, a gain error is present. Once calibration is applied to the measured results from the ADC, the gain error is minimized to nearly zero.
Room Temperature (25°C) Measurements | |||||
---|---|---|---|---|---|
VIN | Code | Uncalibrated VIN | Calibrated VIN | Voltage Error Without Calibration % | Voltage Error With Calibration % |
9.5 | 30841 | 9.412 | 9.500000 | –0.926316 | –0.000001 |
8.5 | 27594 | 8.421 | 8.499789 | –0.929412 | –0.002480 |
5 | 16232 | 16232 | 4.999822 | –0.920000 | –0.003568 |
0 | 1 | 0 | 0.000000 | – | – |
–5 | –16230 | –4.953 | –4.999822 | –0.0940000 | –0.003567 |
–8.5 | –27593 | –8.421 | –8.500097 | –0.929412 | 0.001144 |
–9.5 | –30839 | –9.411 | –9.500000 | –0.936842 | 0.000000 |
When exposed to high temperatures, the gain error increases, as expected. Once calibration is applied, the voltage error is decreased but not eliminated; the error still present is the drift error.
High Temperature (125°C) Measurements | |||||
---|---|---|---|---|---|
VIN | Code | Uncalibrated VIN | Calibrated VIN | Relative Voltage Error Without Calibration % | Relative Voltage Error With Calibration % |
9.5 | 30826 | 9.407 | 9.495379 | –0.978947 | –0.048639 |
8.5 | 27582 | 8.417 | 8.496093 | –0.976471 | –0.045968 |
5 | 16224 | 4.951 | 4.997357 | –0.980000 | –0.052854 |
0 | 0 | 0 | –0.000308 | 0 | – |
–5 | –16224 | –4.951 | –4.997973 | –0.980000 | –0.040531 |
–8.5 | –27581 | –8.417 | –8.496401 | –0.976471 | –0.042344 |
–9.5 | –30826 | –9.407 | –9.495995 | –0.978947 | –0.042153 |
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Device | Key Features | Link | Similar Devices |
---|---|---|---|
ADS8588S | 16-bit, high-speed 8-channel simultaneous-sampling ADC with bipolar inputs on a single supply | www.ti.com/product/ADS8588S | www.ti.com/adcs |
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