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Amplifier permits offset compensation from single supply

19 Jan 2016  | Luca Bruno

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The amplifier's differential gain is 3+(92.8 kΩ/RG), and the reference voltage is added, inverted together with the output signal. Resistor RG sets the gain, and, if you do not connect RG, the gain assumes the minimum value, which is three; decreasing the value of RG to 93Ω increases the gain to 1000.

The VREF input requires a low-impedance connection to preserve a good common-mode-rejection ratio (CMRR) (); otherwise, you can use an op-amp buffer for better CMRR, which depends mainly on resistor-ratio matching. In this implementation, to preserve an acceptable CMRR, you must use precision film resistors. Analysing the circuit, you can calculate the worst-case CMRR at low frequency. With R2, R3, R4, and R5 all of equal value and double that of R1 and with all the resistors having equal tolerance, you obtain:



where ΔR/R is the resistor's tolerance. If the tolerance is 0.1% and with the minimum differential gain, which is three, you obtain a CMRR of at least 54 dB. With a differential gain of 100, you obtain a CMRR of at least 84 dB.

The VREF input can reduce the system offset to the lower output-swing limit but does not reset it completely because, in that case, the output voltage would be unable to reach the single-supply ground. If you want instead to reset the output offset, you can subtract this value using an ADC with differential inputs (Reference).
Reference
Bruno, Luca, "Circuit compensates system offset of a load-cell-based balance," EDN, Aug 16, 2007, pg 71.


About the author
Luca Bruno contributed this article.


This article is a Design Idea selected for re-publication by the editors. It was first published on March 19, 2009 in EDN.com.


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