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ALD2724SB Datasheet(PDF) 2 Page - Advanced Linear Devices |
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ALD2724SB Datasheet(HTML) 2 Page - Advanced Linear Devices |
2 / 13 page ALD2724E/ALD2724 Advanced Linear Devices 2 of 13 FUNCTIONAL DESCRIPTION The ALD2724E/ALD2724 uses EPADs as in-circuit ele- ments for trimming of offset voltage bias characteristics. Each ALD2724E/ALD2724 has a pair of EPAD-based cir- cuits 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 programmable device, the VOS of the ALD2724E can be adjusted many times in both direc- tions. Once programmed, the set VOS levels are stored permanently, even when the device power is removed. Functional Description of ALD2724E The ALD2724E 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 ALD2724E 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 ALD2724E can be trimmed with the actual intended operating conditions. For example, an application circuit may have +5V and -5V power supplies, and the operational amplifier input is biased at +1V, and an average operating temperature at +85 °C. The circuit can be wired up to these conditions within an environmental chamber with the ALD2724E 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 (VOST) is approximately ±25µV for the ALD2724E. In-System Programming refers to the condition where the EPAD adjustment is made after the ALD2724E has been inserted into a circuit board. In this case, the circuit design must provide for the ALD2724E to operate in normal mode and in programming mode. One of the benefits of in-system programming is that not only is the ALD2724E 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. Functional Description of ALD2724 The ALD2724 is pre-programmed at the factory under stan- dard operating conditions for minimum equivalent input off- set voltage. The ALD2724 offers similar programmable features as the ALD2724E, but with a more limited offset voltage program range. In is intended for standard opera- tional amplifier applications, where little or no electrical porggramming by the user is necessary. USER PROGRAMMABLE VOS FEATURE Each ALD2724E/ALD2724 has four additional pins, com- pared to a conventional dual operational amplifier which has eight pins. These four additional pins are named VE1A, VE2A for op amp A and VE1B, VE2B for op amp B. Each of these pins VE1A, VE2A, VE1B, VE2B (represented by VExx) are connected to a separate, internal offset bias circuit. VExx pins have initial internal bias voltage values of approximately 1V to 2V. The voltage on these pins can be programmed using the ALD E100 EPAD Programmer and the appropriate Adapter Module. The useful programming range of voltages on VExx pins are 1V to 4V. VExx pins are programming pins, used during electrical programming mode to inject charge into the internal EPADs. Increasing voltage on VE1A/VE1B decreases the offset voltage whereas increasing voltage on VE2A/VE2B in- creases the offset voltage of op amp A and op amp B, respectively. The injected charge is then permanently stored. After programming, VExx pins must be left open in order for these voltages to remain at the programmed levels. During programming, voltages on VExx pins are increased incrementally to program the offset voltage of the operational amplifier to the desired VOS. Note that desired VOS can be any value within the offset voltage programmable ranges, and can be equal 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 VE1x or VE2x programming voltage range has not been exceeded. VExx 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 ALD2724E 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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