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ADS1210 Datasheet(PDF) 13 Page - Texas Instruments |
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ADS1210 Datasheet(HTML) 13 Page - Texas Instruments |
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13 / 50 page  ADS1210, ADS1211 13 SBAS034B www.ti.com The Turbo Mode Rate (TMR) is programmed via the Sam- pling Frequency bits of the Command Register. Due to the increase in input capacitor sampling frequency, higher Turbo Mode settings result in lower analog input impedance; AIN Impedance (Ω) = (10MHz/fXIN)•4.3E6/(G•TMR) where G is the gain setting. Because the modulator rate also changes in direct relation to the Turbo Mode setting, higher values result in a lower impedance for the REFIN input: REFIN Impedance (Ω) = (10MHz/fXIN)•1E6/TMR The Turbo Mode Rate can be set to 1, 2, 4, 8, or 16. Consult the graphs shown in the Typical Performance Curves for full details on the performance of the ADS1210/11 operating in different Turbo Mode Rates. Keep in mind that higher Turbo Mode Rates result in fewer available gain settings as shown in Table II. PROGRAMMABLE GAIN AMPLIFIER The programmable gain amplifier gain setting is programmed via the PGA Gain bits of the Command Register. Changes in the gain setting (G) of the programmable gain amplifier results in an increase in the input capacitor sampling fre- quency. Thus, higher gain settings result in a lower analog input impedance: AIN Impedance (Ω) = (10MHz/fXIN)•4.3E6/(G•TMR) where TMR is the Turbo Mode Rate. Because the modulator speed does not depend on the gain setting, the input imped- ance seen at REFIN does not change. The PGA can be set to gains of 1, 2, 4, 8, or 16. These gain settings with their resulting full-scale range and typical voltage range are shown in Table I. Keep in mind that higher Turbo Mode Rates result in fewer available gain settings as shown in Table II. SOFTWARE GAIN The excellent performance, flexibility, and low cost of the ADS1210/11 allow the converter to be considered for de- signs which would not normally need a 24-bit ADC. For example, many designs utilize a 12-bit converter and a high- gain INA or PGA for digitizing low amplitude signals. For some of these cases, the ADS1210/11 by itself may be a solution, even though the maximum gain is limited to 16. To get around the gain limitation, the digital result can simply be shifted up by “n” bits in the main controller— resulting in a gain of “n” times G, where G is the gain setting. While this type of manipulation of the output data is obvious, it is easy to miss how much the gain can be increased in this manner on a 24-bit converter. For example, shifting the result up by three bits when the ADS1210/11 is set to a gain of 16 results in an effective gain of 128. At lower data rates, the converter can easily provide more than 12 bits of resolution. Even higher gains are possible. The limitation is a combination of the needed data rate, desired noise performance, and desired linearity. CALIBRATION The ADS1210/11 offers several different types of calibra- tion, and the particular calibration desired is programmed via the Command Register. In the case of Background Calibration, the calibration will repeat at regular intervals indefinitely. For all others, the calibration is performed once and then normal operation is resumed. Each type of calibration is covered in detail in its respective section. In general, calibration is recommended immediately after power-on and whenever there is a “significant” change in the operating environment. The amount of change which should cause a re-calibration is dependent on the applica- tion, effective resolution, etc. Where high accuracy is impor- tant, re-calibration should be done on changes in tempera- ture and power supply. In all cases, re-calibration should be done when the gain, Turbo Mode, or data rate is changed. After a calibration has been accomplished, the Offset Cali- bration Register and the Full-Scale Calibration Register contain the results of the calibration. The data in these registers are accurate to the effective resolution of the ADS1210/11’s mode of operation during the calibration. Thus, these values will show a variation (or noise) equiva- lent to a regular conversion result. For those cases where this error must be reduced, it is tempting to consider running the calibration at a slower data rate and then increasing the converter’s data rate after the calibration is complete. Unfortunately, this will not work as expected. The reason is that the results calculated at the slower data rate would not be valid for the higher data rate. Instead, the calibration should be done repeatedly. After each calibration, the results can be read and stored. After the desired number of calibrations, the main controller can compute an average and write this value into the calibration registers. The resulting error in the calibration values will be reduced by the square root of the number of calibrations which were averaged. The calibration registers can also be used to provide system offset and gain corrections separate from those computed by the ADS1210/11. For example, these might be burned into E2PROM during final product testing. On power-on, the main controller would load these values into the calibration registers. A further possibility is a look-up table based on the current temperature. Note that the values in the calibration registers will vary from configuration to configuration and from part to part. There is no method of reliably computing what a particular calibration register should be to correct for a given amount of system error. It is possible to present the ADS1210/11 with a known amount of error, perform a calibration, read the desired calibration register, change the error value, perform another calibration, read the new value and use these values to interpolate an intermediate value. |
Número de pieza similar - ADS1210_15 |
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Descripción similar - ADS1210_15 |
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