Datasheet LTC1042 (Analog Devices) - 4

LTC1042
U U W U APPLICATIO S I FOR ATIO
The LTC1042 uses sampled data techniques to achieve its storing the results in CMOS output latches and turning the unique characteristics. It consists of two comparators, power off. This whole process takes approximately 80µs. each of which has two differential inputs (Figure 1). When During the 80µs “active” time, the LTC1042 draws the sum of the voltages on a comparator’s inputs is typically 1.2mA (lS(ON)) at V+ = 5V. Because power is positive, the output is high; when the sum is negative, the consumed only during the “active” time, extremely low output is low. The inputs are interconnected such that average power consumption can be achieved at low sample when (CENTER – WIDTH/2) ≤ VIN ≤ (CENTER + WIDTH/2) rates. For example, at a sample rate of 1 sample/second both comparator outputs are low. In this condition VIN is the average power consumption is: within the window and the WITHIN WINDOW output is Power = (V+) (I high. When V S(AVG)) = 5V • 1.2mA • 80µs/1sec IN > CENTER + WIDTH/2, VIN is above the = 0.48µW window and the ABOVE WINDOW output is high. At low sampling rates, R An important feature of the LTC1042 is the non-interaction EXT dominates the power con- sumption. R of the inputs. This means the center and width of the EXT consumes power continuously. The aver- age voltage at the OSC pin is approximately V+/2. The window can be changed without one affecting the other. power consumed by R Also note that the width of the window is set by a ground EXT is: referred signal WlDTH/2). P(REXT) = (V+/2)2REXT Example: Assume R
Strobing
EXT = 1MΩ and V+ = 5V. Then: P(R An internal oscillator allows the LTC1042 to strobe itself. EXT) = (2.5)2/1MΩ = 6.25µW The frequency of oscillation sets the sampling rate and is This is more than ten times the typical power consumed by set with an external RC network (see typical curve, OSC the LTC1042 at V+ = 5V and 1 sample/second. Where frequency vs R power is a premium, R EXT, CEXT). To assure oscillation, under all EXT should be made as large as conditions, R possible. Note that the power dissipated by R EXT must be between 100kΩ and 10MΩ. EXT is not a There is no limit to the size of C function of the sampling frequency or C EXT. EXT. A sampling cycle is initiated on the positive going transi- If high sampling rates are needed and power consumption tion of the voltage on the OSC pin. When this voltage is is of secondary importance, a convenient way to get the near the positive supply, a Schmitt trigger trips and maximum possible sampling rate is to make REXT = 100kΩ initiates the sampling cycle. A sampling cycle consists of and CEXT = 0. The sampling rate, set by the LTC1042’s applying power to both comparators, sampling the inputs, active time, will nominally be ≈ 10kHz. WINDOW 2 + CENTER – 8 V+ (VIN) + COMP A – 4 WINDOW WITHIN ABOVE CENTER WINDOW WINDOW 1 WITHIN WINDOW VIN (WINDOW 3 + V+ CENTER) – ABOVE WINDOW TAGE (V) + COMP B 6 (BELOW WINDOW) WIDTH/2 5 – –WIDTH/2 WIDTH/2 OUTPUT VOL GND 4 4 0V V POWER ON L VU TIMING INPUT VOLTAGE, VIN OSC 7 GENERATOR POWER OFF 80µs
(A) (B)
LTC1042 • AI01
Figure 1. LTC1042 Block Diagram
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