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3650MGHG1 Datasheet(PDF) 6 Page - Burr-Brown (TI) |
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3650MGHG1 Datasheet(HTML) 6 Page - Burr-Brown (TI) |
6 / 13 page 6 ® 3650/52 DEFINITIONS ISOLATION-MODE VOLTAGE, VISO The isolation-mode voltage is the voltage which appears across the isolation barrier, i.e., between the input common and the output common. (See Figure 1.) Two isolation voltages are given in the electrical specifica- tions: “rated continuous” and “test voltage”. Since it is impractical on a production basis to test a “continuous” voltage (infinite test time is implied), it is a generally accepted practice to test at a significantly higher voltage for some reasonable length of time. For the 3650 and 3652, the “test voltage” is equal to 1000V plus two times the “rated continuous” voltage. Thus, for a continuous rating of 2000V, each unit is tested at 5000V. COMMON-MODE VOLTAGE, VCM The common-mode voltage is the voltage midway between the two inputs of the amplifier measured with respect to input common. It is the algebraic average of the voltage applied at the amplifiers’ input terminals. In the circuit in Figure 1, (V+ + V–)/2 = VCM. (NOTE: Many applications involve a large system “common-mode voltage.” Usually in such cases the term defined here as “VCM” is negligible and the system “common-mode voltage” is applied to the ampli- fier as “VISO” in Figure 1.) ISOLATION-MODE REJECTION The isolation-mode rejection is defined by the equation in Figure 1. The isolation-mode rejection is not infinite be- cause there is some leakage across the isolation barrier due to the isolation resistance and capacitance. FIGURE 1. Illustration of Isolation-Mode and Common- Mode Specifications. NONLINEARITY Nonlinearity is specified to be the peak deviation from a best straightline expressed as a percent of peak-to-peak full scale output (i.e. ±10mV at 20Vp-p ≈ 0.05%). THEORY OF OPERATION Prior to the introduction of the 3650 family optical isolation had not been practical in linear circuits. A single LED and photodiode combination, while useful in a wide range of digital isolation applications, has fundamental limitations— primarily nonlinearity and instability as a function of time and temperature. The 3650 and 3652 use a unique technique to overcome the limitations of the single LED and photodiode isolator. Figure 2 is an elementary equivalent circuit for the 3650, which can be used to understand the basic operation without considering the cluttering details of offset adjustment and biasing for bipolar operation. FIGURE 2. Simplified Equivalent Circuit of Linear Isolator. Two matched photodiodes are used—one in the input (CR3) and one in the output stage (CR2)—to greatly reduce nonlinearities and time-temperature instabilities. Amplifier A1, LED CR1, and photodiode CR3 are used in a negative feedback configuration such that I1 = IIN RG (where RG is the user supplied gain setting resistor). Since CR2 and CR3 are closely matched, and since they receive equal amounts of light from the LED CR1 (i.e., λ1 = λ2), I2 = I1 = IIN. Amplifier A2 is connected as a current-to-voltage converter with VOUT = I2 RK where RK is an internal 1MΩ scaling resistor. Thus the overall transfer function is: VOUT = VIN , (RG in Ωs) This improved isolator circuit overcomes the primary limitations of the single LED and photodiode combination. The transfer function is now virtually independent of any degradation in the LED output as long as the two photo- diodes and optics are closely matched(1). Linearity is now a NOTE: (1) The only effect of decreased LED output is a slight decrease in full scale swing capability. See Typical Performance Curves. C (Output) + R G1 R G2 R IN V ISO V CM V D – + V+ V– C (Input) I L – V OUT Isolation Barrier System Ground 10 6 R G1 + RG2 + RIN V CM CMRR V OUT = V D + + V ISO IMRR V IN V OUT = VIN R K R G A 1 R G Input Common I IN +V –V I 1 λ1 λ2 CR 3 CR 1 CR 2 R K A 2 +V CC V OUT –V CC Output Common I 2 I 2 I 3 Isolation Barrier – + – + 106 R G |
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