AD8253 datasheet(24/34 Pages) AD

24 / 34 page

LTC6373

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Dynamic Power Consumption Calculation

As shown in the Simplified Block Diagram of Figure 1, the

LTC6373 has three internal chains of gain setting resis-

tors. To achieve a low wideband noise for the LTC6373, a

relatively small value, 4kΩ, has been chosen for the total

resistance of each chain. The voltages across the three

chains are:

1) VOUTA1 to –OUT

2) VOUTA2 to +OUT

3) VOUTA1 to VOUTA2

Each of these voltages is imposed across what is effec-

tively one 4kΩ resistor, establishing currents in them.

These three currents are independent of each other and

the part’s quiescent supply current (IS), and all of them

are drawn from the supplies.

For example, assume LTC6373 is being used with ±15V

supplies (i.e., V+ = V+OUT = 15V, V– = –15V), VOCM = 0V,

G = 2, and has input voltages of +IN = 3V and –IN = –3V

(i.e., VICM = 0V, VINDIFF = 6V). The resulting output voltage

is VOUTDIFF = 2 • VINDIFF = 12V. Since VOUTCM = VOCM = 0V,

this implies that the value of LTC6373’s output voltages

are +OUT = 6V, –OUT = –6V.

Since the gain is applied in the A1 and A2 amplifiers, the

output voltages of these internal amplifiers are VOUTA1 =

+6V and VOUTA2 = –6V, respectively.

Thus, the voltages and currents in each 4kΩ resistor

chain are:

I1 = [(VOUTA1) – (–OUT)]/4kΩ

= [6V – (–6V)]/4kΩ

= 3mA

I2 = [(+OUT) – (VOUTA2)]/4kΩ

= [6V – (–6V)]/4kΩ

= 3mA

I3 = [(VOUTA1) – (VOUTA2)]/4kΩ

= [6V – (–6V)]/4kΩ

= 3mA

Therefore, the total supply current is:

ITOTAL = IS + I1 + I2 + I3 = 4.4mA + 3 • 3mA = 13.4mA

In case the output pins (+OUT, −OUT) of the LTC6373

connect to resistive loads, the currents provided by the

LTC6373 to these loads should also be added to the cal-

culations above.

Board Layout and Bypass Capacitors

It is recommended that high quality 0.1μF ceramic bypass

capacitors be placed directly between the V+ pin and the

V– pin (exposed pad), between V+ and ground plane, and

between V– and ground plane with minimal routing. In

applications where V+OUT pin is not directly connected to

V+, it is recommended that additional high quality 0.1μF

ceramic capacitors be used to bypass V+OUT to ground

and V+OUT to V–, again with minimal routing. Small geom-

etry (e.g., 0603) surface mount ceramic capacitors have a

much higher self-resonant frequency than leaded capaci-

tors, and perform best with the LTC6373.

Always keep in mind the differential nature of the

LTC6373. At the inputs, keep any (intended or parasitic)

resistance and capacitance as balanced and symmetric

as possible to preserve AC CMRR performance of the

amplifier. Apply the same practice at the output, because

it is equally critical that the load impedances seen by both

outputs (intended or parasitic) be as balanced and sym-

metric as possible. This will help preserve the balanced

operation of the LTC6373 that minimizes the generation

of even-order harmonics and maximizes the rejection of

common mode noise and signals.

To minimize thermocouple induced errors, further atten-

tion must be given to board layout and component selec-

tion. It is good practice to minimize the number of junc-

tions in the LTC6373’s input signal paths and avoid con-

nectors, sockets, switches, and relays whenever possible.

If such components are required, they should be selected

for low thermal EMF characteristics. Furthermore, the

number, type, and layout of junctions should be matched

for both inputs with respect to thermal gradients on the

circuit board. Doing so may involve deliberately introduc-

ing dummy junctions to offset unavoidable junctions.

The VOCM pin should be bypassed to the ground plane with

a high quality 0.1μF ceramic capacitor. This will prevent

common mode signals and noise on this pin from being

inadvertently converted to differential signals and noise by

AD8253 Datasheet(PDF) 24 Page - Analog Devices

AD8253 Datasheet(HTML) 24 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 29 30 31 32 33 34

Datasheet Download

Link URL