Datasheet LT1920 (Analog Devices) - 9

LT1920
U THEORY OF OPERATIO Input and Output Offset Voltage Output Offset Trimming
The offset voltage of the LT1920 has two components: the The LT1920 is laser trimmed for low offset voltage so that output offset and the input offset. The total offset voltage no external offset trimming is required for most applica- referred to the input (RTI) is found by dividing the output tions. In the event that the offset needs to be adjusted, the offset by the programmed gain (G) and adding it to the circuit in Figure 2 is an example of an optional offset adjust input offset. At high gains the input offset voltage domi- circuit. The op amp buffer provides a low impedance to the nates, whereas at low gains the output offset voltage REF pin where resistance must be kept to minimum for dominates. The total offset voltage is: best CMRR and lowest gain error. Total input offset voltage (RTI) 2 – = input offset + (output offset/G) –IN 1 Total output offset voltage (RTO) 6 R LT1920 = (input offset • G) + output offset G OUTPUT V+ REF 8 5
Reference Terminal
3 + +IN – 2 10mV The reference terminal is one end of one of the four 10k 1 1/2 100Ω LT1112 resistors around the difference amplifier. The output volt- ±10mV + 3 10k ADJUSTMENT RANGE age of the LT1920 (Pin 6) is referenced to the voltage on 100Ω the reference terminal (Pin 5). Resistance in series with –10mV the REF pin must be minimized for best common mode rejection. For example, a 2Ω resistance from the REF pin to ground will not only increase the gain error by 0.02% V– 1920 F02 but will lower the CMRR to 80dB.
Figure 2. Optional Trimming of Output Offset Voltage Single Supply Operation Input Bias Current Return Path
For single supply operation, the REF pin can be at the same The low input bias current of the LT1920 (2nA) and the potential as the negative supply (Pin 4) provided the high input impedance (200GΩ) allow the use of high output of the instrumentation amplifier remains inside the impedance sources without introducing additional offset specified operating range and that one of the inputs is at voltage errors, even when the full common mode range is least 2.5V above ground. The barometer application on the required. However, a path must be provided for the input front page of this data sheet is an example that satisfies bias currents of both inputs when a purely differential these conditions. The resistance RSET from the bridge signal is being amplified. Without this path the inputs will transducer to ground sets the operating current for the float to either rail and exceed the input common mode bridge and also has the effect of raising the input common range of the LT1920, resulting in a saturated input stage. mode voltage. The output of the LT1920 is always inside Figure 3 shows three examples of an input bias current the specified range since the barometric pressure rarely path. The first example is of a purely differential signal goes low enough to cause the output to rail (30.00 inches source with a 10kΩ input current path to ground. Since the of Hg corresponds to 3.000V). For applications that re- impedance of the signal source is low, only one resistor is quire the output to swing at or below the REF potential, the needed. Two matching resistors are needed for higher voltage on the REF pin can be level shifted. An op amp is impedance signal sources as shown in the second used to buffer the voltage on the REF pin since a parasitic example. Balancing the input impedance improves both series resistance will degrade the CMRR. The application common mode rejection and DC offset. The need for input in the back of this data sheet, Four Digit Pressure Sensor, resistors is eliminated if a center tap is present as shown is an example. in the third example. 9