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ISL28272 Fiches technique(PDF) 11 Page - Intersil Corporation |
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ISL28272 Fiches technique(HTML) 11 Page - Intersil Corporation |
11 / 14 page 11 FN6390.0 December 8, 2006 Application Information Product Description The ISL28271 and ISL28272 are dual channel micropower instrumentation amplifiers (in-amps) which deliver rail-to-rail input amplification and rail-to-rail output swing. The in-amps also deliver excellent DC and AC specifications while consuming only about 120µA for both channels. Because the independent pair of feedback terminals set the gain and adjust the output zero level, the ISL28271 and ISL28272 achieve high CMRR regardless of the tolerance of the gain setting resistors. The ISL28271 is internally compensated for a minimum gain of 10. The ISL28272 is internally compensated for a minimum gain of 100. EN pins are available to independently enable or disable a channel. When all channels are off, current consumption is down to typically 4µA. Input Protection All input terminals and feedback terminals have internal ESD protection diodes to both positive and negative supply rails, limiting the input voltage to within one diode beyond the supply rails. Input signals originating from low impedance sources should have current limiting resistors in series with the IN+ and IN- pins to prevent damaging currents during power supply sequencing and other transient conditions. The ISL28272 has additional back-to-back diodes across the input terminals and also across the feedback terminals. If overdriving the inputs is necessary, the external input current must never exceed 5mA. External series resistors may be used as an external protection to limit excessive external voltage and current from damaging the inputs. On the other hand, the ISL28271 has no clamps to limit the differential voltage on the input terminals allowing higher differential input voltages at lower gain applications. It is recommended however, that the terminals of the ISL28271 are not overdriven beyond 1V to avoid offset drift. Input Stage and Input Voltage Range The input terminals (IN+ and IN-) of the in-amps are a single differential pair of CMOS devices aided by an Input Range Enhancement Circuit, IREC, to increase the headroom of operation of the common-mode input voltage. The feedback terminals (FB+ and FB-) also have a similar topology. As a result, the input common-mode voltage range is rail-to-rail regardless of the feedback terminal settings and regardless of the gain settings. They are able to handle input voltages that are at or slightly beyond the supply and ground sensing making these in-amps well suited for single 5V down to 2.4V supply systems. The IREC enables rail-to-rail input amplification without the problems usually associated with the dual differential stage topology. The IREC ensures that there are no drastic changes in offset voltage over the entire range of the input. See Input Offset Voltage vs Common-Mode Input Voltage in performance charts. IREC also cures the abrupt change and even reverse polarity of the input bias current over the whole range of input. Output Stage and Output Voltage Range A Class AB common-source output stage drives the output. The pair of complementary MOSFET devices drive the output VOUT to within a few millivolts of the supply rails. At a 100k Ω load, the PMOS sources current and pulls the output up to 4mV below the positive supply. The NMOS sinks current and pulls the output down to 4mV above the negative supply, or ground in the case of a single supply operation. The current sinking and sourcing capability are internally limited to 31mA. When disabled, the outputs are in a high impedance state. Gain Setting VIN, the potential difference across IN+ and IN-, is replicated (less the input offset voltage) across FB+ and FB-. The function of the in-amp is to maintain the differential voltage across FB- and FB+ equal to IN+ and IN-; (FB- - FB+) = (IN+ - IN-). Consequently, the transfer function can be derived. The in-amp gain is set by two external resistors, the feedback resistor RF, and the gain resistor RG. In Figure 37, the FB+ pin and one end of resistor RG are connected to GND. With this configuration, the above gain equation (Equation 1) is only true for a positive swing in VIN; negative input swings will be ignored because the output will be at ground. Reference Connection Unlike a three op-amp in-amp realization, a finite series resistance seen at the REF terminal does not degrade the high CMRR performance eliminating the need for an + - + - IN+ IN- FB+ FB- V+ V- EN 2.4V to 5V RF RG VOUT EN ISL28271 VCM IN+ IN- FIGURE 37. GAIN IS SET BY TWO EXTERNAL RESISTORS, RF AND RG ISL28272 VOUT 1 R F R G -------- + ⎝⎠ ⎜⎟ ⎛⎞ VIN = VIN IN+ IN- – = (EQ. 1) ISL28271, ISL28272 |
Numéro de pièce similaire - ISL28272 |
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Description similaire - ISL28272 |
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