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LTC6085CDHC Datasheet(PDF) 10 Page - Linear Integrated Systems |
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LTC6085CDHC Datasheet(HTML) 10 Page - Linear Integrated Systems |
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10 / 16 page  LTC6084/LTC6085 10 60845fa APPLICATIONS INFORMATION Rail-to-Rail Input The input stage of LTC6084/LTC6085 combines both PMOS and NMOS differential pairs, extending its input common mode voltage to both positive and negative supply volt- ages. At high input common mode range, NMOS pair is on. At low common mode range, the PMOS pair is on. The transition happens when the common voltage is between 1.3 and 0.9V below the positive supply. Achieving Low Input Bias Current The DD and DHC packages are leadless and make contact to the PCB beneath the package. Solder flux used during the attachment of the part to the PCB can create leakage current paths and can degrade the input bias current per- formance of the part. All inputs are susceptible because the backside paddle is connected to V– internally. As the input voltage or V– changes, a leakage path can be formed and alter the observed input bias current. For lowest bias current use the LTC6084/LTC6085 in the leaded MSOP/GN package. With fine PCB design rules, you can also provide a guard ring around the inputs. For example, in high source impedance applications such as pH probes, photo diodes, strain gauges, etc., the low input bias current of these parts requires a clean board layout to minimize additional leakage current into a high impedance signal node. A mere 100GΩ of PC board resistance between a 5V supply trace and input trace near ground potential adds 50pA of leakage current. This leakage is far greater than the bias current of the operational amplifier. A guard ring around the high impedance input traces driven by a low impedance source equal to the input voltage prevents such leakage problems. The guard ring should extend as far as necessary to shield the high impedance signal from any and all leakage paths. Figure 1 shows the use of a guard ring in a unity-gain configuration. In this case the guard ring is connected to the output and is shielding the high impedance noninverting input from V–. Figure 2 shows the inverting gain configuration. Rail-to-Rail Output The output stage of the LTC6084/LTC6085 swings within 5mV of the supply rails when driving high impedance loads, in other words when no DC load current is present. See the Typical Performance Characteristics for curves of output swing versus load current. The class AB design of the output stage enables the op amp to supply load cur- rents which are much greater than the quiescent supply current. For example, the room temperature short-circuit current is typically 12.5mA. Capacitive Load LTC6084/LTC6085 can drive a capacitive load up to 300pF in unity gain. The capacitive load driving capability increases as the amplifier is used in higher gain configurations. A small series resistance between the output and the load further increases the amount of capacitance the amplifier can drive. Figure 1. Sample Layout. Unity-Gain Configuration. Using Guard Ring to Shield High Impedance Input from Board Leakage Figure 2. Sample Layout. Inverting Gain Configuration. Using Guard Ring to Shield High Impedance Input from Board Leakage LTC6084 R OUT IN– IN+ V– LEAKAGE CURRENT NO LEAKAGE CURRENT GUARD RING NO SOLDER MASK OVER THE GUARD RING 60845 F01 LTC6084 60845 F02 R R OUT IN– IN+ V– VIN GND |
Similar Part No. - LTC6085CDHC |
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Similar Description - LTC6085CDHC |
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