Datasheet LTC6800 (Analog Devices) - 8

LTC6800 block DiagraM 8 V+ +IN 3 + C OUT H –IN CS 7 2 – REF RG V– 5 6 4 6800 BD applicaTions inForMaTion
Theory of Operation
Where V+IN and V–IN are the voltages of the +IN and –IN The LTC6800 uses an internal capacitor (C pins, respectively, VREF is the voltage at the REF pin and S) to sample a differential input signal riding on a DC common mode V+ is the positive supply voltage. voltage (see the Block Diagram). This capacitor’s charge is For example, with a 3V single supply and a 0V to 100mV transferred to a second internal hold capacitor (CH) trans- differential input voltage, VREF must be between 0V and lating the common mode of the input differential signal to 1.6V. that of the REF pin. The resulting signal is amplified by a zero-drift op amp in the noninverting configuration. The
Settling Time
RG pin is the negative input of this op amp and allows The sampling rate is 3kHz and the input sampling period external programmability of the DC gain. Simple filtering during which C can be realized by using an external capacitor across the S is charged to the input differential voltage V feedback resistor. IN is approximately 150µs. First assume that on each input sampling period, CS is charged fully to VIN. Since C
Input Voltage Range
S = CH (= 1000pF), a change in the input will settle to N bits of accuracy at the op amp noninverting input after The input common mode voltage range of the LTC6800 N clock cycles or 333µs(N). The settling time at the OUT is rail-to-rail. However, the following equation limits the pin is also affected by the settling of the internal op amp. size of the differential input voltage: Since the gain bandwidth of the internal op amp is typically V– ≤ (V 200kHz, the settling time is dominated by the switched +IN – V–IN) + VREF ≤ V+ – 1.3 capacitor front end for gains below 100 (see the Typical Performance Characteristics section). 6800fb Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Revision History Related Parts Typical Application