LT1102 UUWUAPPLICATIO S I FOR ATIO In the two op amp instrumentation amplifier configura- Common Mode Rejection Ratio tion, the first amplifier is basically in unity gain, and the vs Frequency second amplifier provides all the voltage gain. In the 120 LT1102, the second amplifier is decompensated for gain G = 100 100 18pF PIN 1 of 10 stability, therefore high slew rate and bandwidth are TO PIN 2 G = 10 achieved. Common mode rejection versus frequency 80 G = 100 is also optimized in the G = 10 mode, because the 60 bandwidths of the two op amps are similar. When G = 100, this statement is no longer true; however, by connecting 40 an 18pF capacitor between pins 1 and 2, a common mode 20 AC gain is created to cancel the inherent roll-off. From V COMMON MODE REJECTION RATIO (dB) S = ±15V TA = 25°C 200Hz to 30kHz, CMRR versus frequency is improved by 0 1 10 100 1k 10k 100k 1M an order of magnitude. FREQUENCY (Hz) LT1102 • AI01 Input Protection Instrumentation amplifiers are often used in harsh Gains Between 10 and 100 environments where overload conditions can occur. Gains between 10 and 100 can be achieved by connecting The LT1102 employs FET input transistors, consequently two equal resistors (= R the differential input voltage can be ±30V (with ±15V X) between pins 1 and 2 and pins 7 and 8. supplies, ±36V with ±18V supplies). Some competitive instrumentation amplifiers have NPN inputs which are RX Gain = 10 + protected by back-to-back diodes. When the differential R + RX/90 input Voltage exceeds ±13V on these competitive devices, input current increases to milliampere level; more than The nominal value of R is 1.84kΩ. The usefulness of this ±10V differential voltage can cause permanent damage. method is limited by the fact that R is not controlled to better than ±10% absolute accuracy in production. When the LT1102 inputs are pulled below the negative However, on any specific unit, 90R can be measured supply or above the positive supply, the inputs will clamp between Pins 1 and 2. a diode voltage below or above the supplies. No damage will occur if the input current is limited to 20mA. 1102fb 8
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