AD8421 datasheet(21/28 Pages) AD

21 / 28 page

LT6372-1

Rev. 0

For more information www.analog.com

10k

6372-1 F07

THERMOCOUPLE

200k

MICROPHONE,

HYDROPHONE,

ETC

200k

CENTER-TAP PROVIDES

BIAS CURRENT RETURN

LT6372-1

REF1,2

Reducing Board-Related Leakage Effects

Leakage currents can have a significant impact on system

accuracy, particularly in high temperature and high voltage

applications. Quality insulation materials should be used,

and insulating surfaces should be cleaned to remove fluxes

and other residues. For humid environments, surface coat-

ing may be necessary to provide a moisture barrier.

Leakage into the RG pin conducts through the on-chip feed-

back resistor, creating an error at the output of the pre-

amplifiers. This error is independent of gain and degrades

accuracy the most at low gains. This leakage can be mini-

mized by encircling the RG connections with a guard-ring

operated at a potential very close to that of the RG pins.

NIC pins adjacent to each RG pin can be used to simplify

the implementation of this guard-ring. These NIC pins do

not provide any bias and have no internal connections. In

some cases, the guard-ring can be connected to the input

voltage which biases one diode drop below RG.

Figure 5.

6372-1 F05

LT6372-1

–

V–

V+

Guard-Rings Can Be Used to Minimize

Leakage into the RG Pins

Leakage into the input pins reacts with the source resis-

tance, creating an error directly at the input. This leakage

can be minimized by encircling the input connections with

a guard-rings operated at a potential very close to that

of the input pins. In some cases, the guard-ring can be

connected to RG which biases one diode above the input.

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.

Figure 6.

6372-1 F06

LT6372-1

–

V–

V+

Guard-Rings Can Be Used to Minimize

Leakage into the Input Pins

Input Bias Current Return Path

The low input bias current of the LT6372-1 (800pA max) and

high input impedance (225GΩ) allow the use of high imped-

ance 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 cur-

rents 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 LT6372-1,

resulting in a saturated input amplifier. Figure 7 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 imped-

ance 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 7. Providing an Input Common Mode Current Path

AD8421 датащи(PDF) 21 Page - Analog Devices

AD8421 датащи(HTML) 21 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

датащи скачать

ссылки URL