Datasheet LTC1100 (Analog Devices) - 7

LTC1100
U U W U APPLICATIO S I FOR ATIO Common Mode Rejection Aliasing
Due to very precise matching of the internal resistors, no The LTC1100 is a chopper-stabilized instrumentation trims are required to obtain a DC CMRR of better than amplifier; like all sampled systems it exhibits aliasing 100dB; however, things change as frequency rises. The behavior for input frequencies at or near the internal inverting amplifier is in a gain of 1.01 (1.1 for gain of 10), sampling frequency. The LTC1100 incorporates special- while the noninverting amplifier is in a gain of 99 (9 for gain ized anti-aliasing circuitry which typically attenuates of 10). As frequency rises, the higher gain amplifier hits its aliasing products by ≥ 60dB; however, extremely sensi- gain-bandwidth limit long before the low gain amplifier, tive systems may still have to take precautions to avoid degrading CMRR. The solution is straightforward — slow aliasing errors. For more information, see the LTC1051/ down the inverting amplifier to match the noninverting LTC1053 data sheet. amp. Figure 1 shows the recommended circuit. The prob-
Single Supply Operation
lem is less pronounced in the LTC1100CS in gain-of-10 mode; no CMRR trims are necessary. The LTC1100 will operate on a single 5V supply, and the common mode range of the internal op amps includes 3 − − ground; single supply operation is limited only by the 8 output swing of the op amps. The internal inverting LTC1100 6 + + amplifier has a negative saturation limit of 5mV typically, 2 setting the minimum common mode limit at 5mV/1.01 (or 100k 1.1 for gain of 10). The inputs can be biased above ground, 10pF as shown in Figure 3. Low cost biasing components can be used since any errors appear as a common mode term and LTC1100 • TA02 are rejected.
Figure 1. Improving AC CMRR
The minimum differential input voltage is limited by the
Overcompensation
swing of the output op amp. Lightly loaded, it will swing down to 5mV, allowing differential input voltages as low as Many instrumentation amplifier applications process DC 50µV (450µV for gain of 10). Single supply operation or low frequency signals only; consequently, the 18kHz limits the LTC1100 to positive differential inputs only; (180kHz for G = 10) bandwidth of the LTC1100 can be negative inputs will give a saturated zero output. reduced to minimize system errors or reduce transmitted clock noise by using the COMP pin. A feedback cap from 5V COMP to VOUT will react with the 247k internal resistor 5V (22.5k for G = 10) to limit the bandwidth, as in Figure 2. RBIAS 6 5 CB + 8 OUTPUT SENSOR LTC1100 3 0V TO 5V − 4, 1 3 7 – 8 LTC1100 1N4148 6 + 1 f = 3dB LTC1100 • TA04 2 R π INT × CB R = 247k FOR G = 100 INT 22.5k FOR G = 10
Figure 3
LTC1100 • TA03
Figure 2. Overcompensation to Reduce System Bandwidth
1100fc Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen- tation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 7