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LTC2499 Datasheet(PDF) 24 Page - Linear Technology |
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LTC2499 Datasheet(HTML) 24 Page - Linear Technology |
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24 / 32 page  LTC2499 24 2499fb input impedances, the common mode input current effects are rejected by the large CMRR of the LTC2499, leading to little degradation in accuracy. Mismatches in source impedances lead to gain errors proportional to the dif- ference between the common mode input and common mode reference. 1% mismatches in 1k source resistances lead to gain errors on the order of 15ppm. Based on the stability of the internal sampling capacitors and the ac- curacy of the internal oscillator, a one-time calibration will remove this error. In addition to the input sampling current, the input ESD protection diodes have a temperature dependent leakage current. This current, nominally 1nA (±10nA max), results in a small offset shift. A 1k source resistance will create a 1μV typical and a 10μV maximum offset voltage. Automatic Offset Calibration of External Buffers/ Amplifiers In addition to the Easy Drive input current cancellation, the LTC2499 allows an external amplifier to be inserted between the multiplexer output and the ADC input (see Figure 12). This is useful in applications where balanced source impedances are not possible. One pair of external buffers/amplifiers can be shared between all 17 analog inputs. The LTC2499 performs an internal offset calibration every conversion cycle in order to remove the offset and drift of the ADC. This calibration is performed through a combination of front end switching and digital process- ing. Since the external amplifier is placed between the multiplexer and the ADC, it is inside this correction loop. This results in automatic offset correction and offset drift removal of the external amplifier. The LTC6078 is an excellent amplifier for this function. It operates with supply voltages as low as 2.7V and its noise level is 18nV/√Hz. The Easy Drive input technology of the LTC2499 enables an RC network to be added directly to the output of the LTC6078. The capacitor reduces the magnitude of the current spikes seen at the input to the ADC and the resistor isolates the capacitor load from the op-amp output enabling stable operation. The LTC6078 can also be biased at supply rails beyond those used by the LTC2499. This allows the external sensor to swing rail- to-rail (–0.3V to VCC + 0.3V) without the need of external level shift circuitry. Figure 12. External Buffers Provide High Impedance Inputs and Amplifier Offsets are Automatically Cancelled APPLICATIONS INFORMATION – + – + 1/2 LTC6078 1/2 LTC6078 1 2 3 5 6 7 ΔΣ ADC WITH EASY DRIVE INPUTS INPUT MUX 17 LTC2499 ANALOG INPUTS SCL SDA 0.1μF 1kΩ 1kΩ 0.1μF |
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