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AD8208WBRZ-RL Datasheet(PDF) 10 Page - Analog Devices |
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AD8208WBRZ-RL Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page AD8208 Rev. A | Page 10 of 16 THEORY OF OPERATION The AD8208 is a single-supply difference amplifier typically used to amplify a small differential voltage in the presence of rapidly changing, high common-mode voltages. The AD8208 consists of two amplifiers (A1 and A2), a resistor network, a small voltage reference, and a bias circuit (not shown); see Figure 24. The set of input attenuators preceding A1 consists of RA, RB, and RC, which feature a combined series resistance of approximately 400 kΩ ± 20%. The purpose of these resistors is to attenuate the input voltage to match the input voltage range of A1. This balanced resistor network attenuates the common-mode signal by a ratio of 1/14. The A1 amplifier inputs are held within the power supply range, even as Pin 1 and Pin 8 exceed the supply or fall below the common (ground). A reference voltage of 350 mV biases the attenuator above ground, allowing Amplifier A1 to operate in the presence of negative common-mode voltages. The input resistor network also attenuates normal (differential) mode voltages. Therefore, A1 features a gain of 140 V/V to provide a total system gain, from ±IN to the output of A1, equal to 10 V/V, as shown in the following equation: Gain (A1) = 1/14 (V/V) × 140(V/V) = 10 V/V A precision trimmed, 100 kΩ resistor is placed in series with the output of Amplifier A1. The user has access to this resistor via an external pin (A1). A low-pass filter can be easily implemented by connecting A1 to A2 and placing a capacitor to ground (see Figure 33). The value of RF1 and RF2 is 10 kΩ, providing a gain of 2 V/V for Amplifier A2. When connecting Pin A1 and Pin A2 together, the AD8208 provides a total system gain equal to Total Gain of (A1 + A2) (V/V) = 10 (V/V) × 2 (V/V) = 20 V/V at the output of A2 (the OUT pin). The ratios of RA, RB, RC, and RF are trimmed to a high level of precision, allowing a typical CMRR value that exceeds 80 dB. This performance is accomplished by laser trimming the resistor ratio matching to better than 0.01%. OUT GND VS RC RF –IN 350mV + – RB RB RA RA RC RF RG RFILTER RM A1 A2 +IN A1 A2 + – RF1 RF2 Figure 24. Simplified Schematic |
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