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ALD1722EPAL Datasheet(PDF) 2 Page - Advanced Linear Devices |
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ALD1722EPAL Datasheet(HTML) 2 Page - Advanced Linear Devices |
2 / 13 page ALD1722E Advanced Linear Devices 2 of 13 FUNCTIONAL DESCRIPTION The ALD1722E uses EPADs as in-circuit elements for trim- ming of offset voltage bias characteristics. Each ALD1722E has a pair of EPAD-based circuits connected such that one circuit is used to adjust VOS in one direction and the other circuit is used to adjust VOS in the other direction. While each of the EPAD devices is a monotonically adjustable program- mable device, the VOS of the ALD1722E can be adjusted many times in both directions. Once programmed, the set VOS levels are stored permanently, even when the device power is removed. The ALD1722E is pre-programmed at the factory under standard operating conditions for minimum equivalent input offset voltage. It also has a guaranteed offset voltage program range, which is ideal for applications that require electrical offset voltage programming. The ALD1722E is an operational amplifier that can be trimmed with user application-specific programming or in- system programming conditions. User application-specific circuit programming refers to the situation where the Total Input Offset Voltage of the ALD1722E can be trimmed with the actual intended operating conditions. For example, an application circuit may have +6V and -2.5V power supplies, and the operational amplifier input is biased at +0.7V, and an average operating temperature at 55 °C. The circuit can be wired up to these conditions within an environmental chamber with the ALD1722E inserted into a test socket connected to this circuit while it is being electri- cally trimmed. Any error in VOS due to these bias conditions can be automatically zeroed out. The Total VOS error is now limited only by the adjustable range and the stability of VOS, and the input noise voltage of the operational amplifier. Therefore, this Total VOS error now includes VOS as VOS is traditionally specified; plus the VOS error contributions from PSRR, CMRR, TCVOS, and noise. Typically this total VOS error term (VOST) is approximately ±25µV for the ALD1722E. The VOS contribution due to PSRR, CMRR, TCVOS and external components can be large for operational amplifiers without trimming. Therefore the ALD1722E with EPAD trim- ming is able to provide much improved system performance by reducing these other sources of error to provide signifi- cantly reduced VOST. In-System Programming refers to the condition where the EPAD adjustment is made after the ALD1722E has been inserted into a circuit board. In this case, the circuit design must provide for the ALD1722E to operate in normal mode and in programming mode. One of the benefits of in-system programming is that not only is the ALD1722E offset voltage from operating bias conditions accounted for, any residual errors introduced by other circuit components, such as resistor or sensor induced voltage errors, can also be cor- rected. In this way, the “in-system” circuit output can be adjusted to a desired level, eliminating the need for another trimming function. USER PROGRAMMABLE VOS FEATURE Each ALD1722E has two pins named VE1 and VE2 which are internally connected to an internal offset bias circuit. VE1/ VE2 have initial typical values of 1.6V. The voltage on these pins can be programmed using the ALD E100 EPAD Pro- grammer and the appropriate Adapter Module. The useful programming range of VE1 and VE2 is 1.6V to 3.5V. VE1 and VE2 pins are programming pins, used during programming mode to inject charge into the internal EPADs. Increasing voltage on VE1 decreases the offset voltage whereas increasing voltage on VE2 increases the offset voltage of the operational amplifier. The injected charge is permanently stored and determines the offset voltage of the operational amplifier. After programming, VE1 and VE2 terminals must be left open to settle on a voltage determined by internal bias currents. During programming, the voltages on VE1 or VE2 are in- creased incrementally to set the offset voltage of the opera- tional amplifier to the desired Vos. Note that desired Vos can be any value within the offset voltage programmable ranges, and can be zero, a positive value or a negative value. This VOS value can also be reprogrammed to a different value at a later time, provided that the useful VE1 or VE2 program- ming voltage range has not been exceeded. VE1 or VE2 pins can also serve as capacitively coupled input pins. Internally, VE1 and VE2 are programmed and connected differentially. Temperature drift effects between the two internal offset bias circuits cancel each other and introduce less net temperature drift coefficient change than offset voltage trimming techniques such as offset adjustment with an external trimmer potentiometer. While programming, V+, VE1 and VE2 pins may be alter- nately pulsed with 12V (approximately) pulses generated by the EPAD Programmer. In-system programming requires the ALD1722E application circuit to accommodate these programming pulses. This can be accomplished by adding resistors at certain appropriate circuit nodes. For more information, see Application Note AN1700. |
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