AD62410k ⍀ 1k ⍀ 10k ⍀ 1%10T1%+VVINPUTSOUT100k ⍀ 20V p-p1%RG1G = 100G = 200AD624G = 5001k ⍀ 500 ⍀ 200 ⍀ 0.1%0.1%0.1%RG2–VS Figure 25. Settling Time Test Circuit THEORY OF OPERATION The AD524 should be considered in applications that require The AD624 is a monolithic instrumentation amplifier based on protection from severe input overload. If this is not possible, a modification of the classic three-op-amp instrumentation external protection resistors can be put in series with the inputs amplifier. Monolithic construction and laser-wafer-trimming of the AD624 to augment the internal (50 Ω) protection resis- allow the tight matching and tracking of circuit components and tors. This will most seriously degrade the noise performance. the high level of performance that this circuit architecture is ca- For this reason the value of these resistors should be chosen to pable of. be as low as possible and still provide 10 mA of current limiting A preamp section (Q1–Q4) develops the programmed gain by under maximum continuous overload conditions. In selecting the use of feedback concepts. Feedback from the outputs of A1 the value of these resistors, the internal gain setting resistor and and A2 forces the collector currents of Q1–Q4 to be constant the 1.2 volt drop need to be considered. For example, to pro- thereby impressing the input voltage across R tect the device from a continuous differential overload of 20 V G. at a gain of 100, 1.9 kΩ of resistance is required. The internal The gain is set by choosing the value of RG from the equation, gain resistor is 404 Ω; the internal protect resistor is 100 Ω. 40 k There is a 1.2 V drop across D1 or D2 and the base-emitter Gain = + 1. The value of RG also sets the transconduct- RG junction of either Q1 and Q3 or Q2 and Q4 as shown in Figure ance of the input preamp stage increasing it asymptotically to 27, 1400 Ω of external resistance would be required (700 Ω in the transconductance of the input transistors as RG is reduced series with each input). The RTI noise in this case would be for larger gains. This has three important advantages. First, this approach allows the circuit to achieve a very high open loop gain 4 KTR +(4 nV / Hz)2 = 6.2 nV / Hz ext of 3 × 108 at a programmed gain of 1000 thus reducing gain +VS related errors to a negligible 3 ppm. Second, the gain bandwidth product which is determined by C3 or C4 and the input trans- I1I2R5250 AVB50 A conductance, reaches 25 MHz. Third, the input voltage noise 10k ⍀ SENSE reduces to a value determined by the collector current of the A1A2C3 input transistors for an RTI noise of 4 nV/√Hz at G ≥ 500. C4R5310k ⍀ +VSR57A3VO50 ⍀ 20k ⍀ R56R54Q1, Q3Q2,+VS20k–INRG ⍀ Q410k ⍀ 1001RG2R5516.2k ⍀ 10k ⍀ 20080.2 ⍀ AD6241 F13REF1/250 A500I4500AD71212450 ⍀ A1/2RG1 F29.09k ⍀ AD7124445 ⍀ 20050 ⍀ +IN16.2k ⍀ 225.3 ⍀ G500G1, 100, 2001 F100–VS1k ⍀ –VS–V100 ⍀ S1.62M ⍀ 1.82k ⍀ Figure 27. Simplified Circuit of Amplifier; Gain Is Defined as (R56 + R57)/(RG) + 1. For a Gain of 1, RG Is an Open Figure 26. Noise Test Circuit Circuit. INPUT CONSIDERATIONSINPUT OFFSET AND OUTPUT OFFSET Under input overload conditions the user will see RG + 100 Ω Voltage offset specifications are often considered a figure of and two diode drops (~1.2 V) between the plus and minus merit for instrumentation amplifiers. While initial offset may inputs, in either direction. If safe overload current under all be adjusted to zero, shifts in offset voltage due to temperature conditions is assumed to be 10 mA, the maximum overload variations will cause errors. Intelligent systems can often correct voltage is ~ ± 2.5 V. While the AD624 can withstand this con- for this factor with an autozero cycle, but there are many small- tinuously, momentary overloads of ± 10 V will not harm the signal high-gain applications that don’t have this capability. device. On the other hand the inputs should never exceed the Voltage offset and offset drift each have two components; input supply voltage. and output. Input offset is that component of offset that is REV. C –7–