Datasheet AD624 (Analog Devices) - 10

AD624 GROUNDING
“inside the loop” of an instrumentation amplifier to provide the Many data-acquisition components have two or more ground required current without significantly degrading overall perfor- pins which are not connected together within the device. These mance. The effects of nonlinearities, offset and gain inaccuracies grounds must be tied together at one point, usually at the sys- of the buffer are reduced by the loop gain of the IA output tem power supply ground. Ideally, a single solid ground would amplifier. Offset drift of the buffer is similarly reduced. be desirable. However, since current flows through the ground wires and etch stripes of the circuit cards, and since these paths
REFERENCE TERMINAL
have resistance and inductance, hundreds of millivolts can be The reference terminal may be used to offset the output by up generated between the system ground point and the data acqui- to ± 10 V. This is useful when the load is “floating” or does not sition components. Separate ground returns should be provided share a ground with the rest of the system. It also provides a to minimize the current flow in the path from the most sensitive direct means of injecting a precise offset. It must be remem- points to the system ground point. In this way supply currents bered that the total output swing is ± 10 volts, from ground, to and logic-gate return currents are not summed into the same be shared between signal and reference offset. return path as analog signals where they would cause measure- ment errors (see Figure 34).
+VS SENSE VIN+ ANALOG P.S. DIGITAL P.S. +15V C –15V C +5V AD624 REF LOAD VIN– 0.1 0.1 0.1 0.1
␮
F
␮
F
␮
F
␮
F 1
␮
F 1
␮
F 1
␮
F –VS V DIG AD711 OFFSET + COM AD583 DIGITAL AD624
Figure 36. Use of Reference Terminal to Provide Output
SAMPLE AD574A DATA
Offset
AND HOLD OUTPUT ANALOG GROUND* SIGNAL
When the IA is of the three-amplifier configuration it is neces-
OUTPUT GROUND REFERENCE
sary that nearly zero impedance be presented to the reference
*IF INDEPENDENT, OTHERWISE RETURN AMPLIFIER REFERENCE
terminal. Any significant resistance, including those caused by
TO MECCA AT ANALOG P.S. COMMON
PC layouts or other connection techniques, which appears Figure 34. Basic Grounding Practice between the reference pin and ground will increase the gain of Since the output voltage is developed with respect to the poten- the noninverting signal path, thereby upsetting the common- tial on the reference terminal an instrumentation amplifier can mode rejection of the IA. Inadvertent thermocouple connections solve many grounding problems. created in the sense and reference lines should also be avoided as they will directly affect the output offset voltage and output
SENSE TERMINAL
offset voltage drift. The sense terminal is the feedback point for the instrument In the AD624 a reference source resistance will unbalance the amplifier’s output amplifier. Normally it is connected to the CMR trim by the ratio of 10 kΩ/RREF. For example, if the refer- instrument amplifier output. If heavy load currents are to be ence source impedance is 1 Ω, CMR will be reduced to 80 dB drawn through long leads, voltage drops due to current flowing (10 kΩ/1 Ω = 80 dB). An operational amplifier may be used to through lead resistance can cause errors. The sense terminal can provide that low impedance reference point as shown in Figure be wired to the instrument amplifier at the load thus putting the 36. The input offset voltage characteristics of that amplifier will IxR drops “inside the loop” and virtually eliminating this error add directly to the output offset voltage performance of the source. instrumentation amplifier. An instrumentation amplifier can be turned into a voltage-to-
V+ (SENSE)
current converter by taking advantage of the sense and reference
OUTPUT
terminals as shown in Figure 37.
CURRENT VIN+ BOOSTER SENSE AD624 X1 +INPUT R R L 1 V +V IN– X– (REF) I AD624 L V– AD711 –INPUT
Figure 35. AD624 Instrumentation Amplifier with Output
REF A2
Current Booster
VX VIN 40.000 LOAD
Typically, IC instrumentation amplifiers are rated for a full
I 1 + L = = R R1 R
±
1 G
10 volt output swing into 2 kΩ. In some applications, how- ever, the need exists to drive more current into heavier loads. Figure 37. Voltage-to-Current Converter Figure 35 shows how a current booster may be connected –10– REV. C