Datasheet LTC6084, LTC6085 (Analog Devices) - 10

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