LT6370 datasheet(20/28 Pages) AD

20 / 28 page

LT6370

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

For the lowest leakage, amplifiers can be used to drive the

guard ring. These buffers must have very low input bias

current since that will now be a leakage.

Input Bias Current Return Path

The low input bias current of the LT6370 (400pA max)

and high input impedance (225GΩ) allow the use of high

impedance sources without introducing additional offset

voltage errors, even when the full common mode range is

required. However, a path must be provided for the input

bias currents of both inputs when a purely differential

signal is being amplified. Without this path, the inputs

will float to either rail and exceed the input common

mode range of the LT6370, resulting in a saturated input

amplifier. Figure 8 shows three examples of an input bias

current path. The first example is of a purely differential

signal source with a 10kΩ input current path to ground.

Since the impedance of the signal source is low, only one

resistor is needed. Two matching resistors are needed for

higher impedance signal sources as shown in the second

example. Balancing the input impedance improves both

AC and DC common mode rejection and DC offset. The

need for input resistors is eliminated if a center tap is

present as shown in the third example.

Figure 8. Providing an Input Common Mode Current Path

10k

6370 F08

THERMOCOUPLE

200k

MICROPHONE,

HYDROPHONE,

ETC

200k

CENTER-TAP PROVIDES

BIAS CURRENT RETURN

LT6370

REF

Input Protection

Additional input protection can be achieved by adding

external resistors in series with each input. If low value

resistors are needed, a clamp diode from the positive

supply to each input will help improve robustness. A

2N4394 drain/source to gate is a good low leakage diode

which can be used as shown in Figure 9. Robust input

resistors should be chosen, such as carbon composite

or bulk metal foil. Metal film and carbon film should not

be used because of their poor performance.

Figure 9. Input Protection

VEE

6370 F05

VCC

2N4393

OUT

OPTIONAL FOR HIGHEST

ESD PROTECTION

RIN

LT6370

REF

Maintaining AC CMRR

To achieve optimum AC CMRR, it is important to balance

the capacitance on the RG gain setting pins. Furthermore,

if the source resistance on each input is not equal, add-

ing an additional resistance to one input to improve input

source resistance matching will improve AC CMRR.

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