MAX4208AUA+T Datasheet MAX4209HAUA+T, MAX4208AUA+T, MAX4209HAUA+ | P13-P15

MAX4208/MAX4209

Ultra-Low Offset/Drift, Precision

Instrumentation Amplifiers with REF Buffer

______________________________________________________________________________________ 13

Applications Information

Setting the Gain (MAX4208)

Connect a resistive divider from OUT to REF with the cen-

ter tap connected to FB to set the gain for the MAX4208

(see the Typical Application Circuit). Calculate the gain

using the following formula:

Choose a value for R1 ≤ 1kΩ. Resistor accuracy ratio

directly affects gain accuracy. Resistor sum less than

10kΩ should not be used because their loading can

slightly affect output accuracy.

Input Common Mode vs.

Input Differential-Voltage Range

Traditional three-op amp instrumentation amplifiers

have a defined relationship between the maximum

input differential voltage and maximum input common-

mode voltage that arises from saturation of intermediate

amplifier stages. This correlation is frequently repre-

sented as a hexagon graph of input common-mode

voltage vs. output voltage for the instrumentation ampli-

fier shown in Figure 3. Application limitations hidden in

this graph are:

• The input common-mode voltage range does not

include the negative supply rail, and so no amplifi-

cation is possible for inputs near ground for single-

supply applications.

• Input differential voltages can be amplified with

maximum gain only over a limited range of input

common-mode voltages (i.e., range of y-axis for max

range of x-axis is limited).

• If large amplitude common-mode voltages need to

be rejected, differential voltages cannot be amplified

with a maximum gain possible (i.e., range of x-axis

for a maximum range of y-axis is limited). As a con-

sequence, a secondary high-gain amplifier is

required to follow the front-end instrumentation

amplifier.

The indirect current-feedback architecture of the

MAX4208/MAX4209 instrumentation amplifiers do not

suffer from any of these drawbacks. Figure 4 shows the

input common-mode voltage vs. output voltage graph

of indirect current-feedback architecture.

In contrast to three-op amp instrumentation amplifiers,

the MAX4208/MAX4209 features:

• The input common-mode voltage range, which

includes the negative supply rail and is ideal for sin-

gle-supply applications.

• Input differential voltages that can be amplified with

maximum gain over the entire range of input com-

mon-mode voltages.

• Large common-mode voltages that can be rejected

at the same time differential voltages are amplified

with maximum gain, and therefore, no secondary

amplifier is required to follow the front-end instru-

mentation amplifier.

Gain Error Drift Over Temperature

Adjustable gain instrumentation amplifiers typically use a

single external resistor to set the gain. However, due to

differences in temperature drift characteristics between

the internal and external resistors, this leads to large

gain-accuracy drift over temperature. The MAX4208 is

an adjustable gain instrumentation amplifier that uses

two external resistors to set its gain. Since both resistors

are external to the device, layout and temperature coeffi-

cient matching of these parts deliver a significantly more

stable gain over operating temperatures.

The fixed gain, MAX4209H has both internal resistors for

excellent matching and tracking.

Use of External Capacitor C

for Noise Reduction

Zero-drift chopper amplifiers include circuitry that con-

tinuously compensates the input offset voltage to deliver

precision and ultra-low temperature drift characteristics.

This self-correction circuitry causes a small additional

noise contribution at its operating frequency (a psuedo-

random clock around 45kHz for MAX4208/MAX4209).

For high-bit resolution ADCs, external filtering can signif-

icantly attenuate this additional noise. Simply adding a

feedback capacitor (C

) between OUT and FB

reduces high-frequency gain, while retaining the excel-

lent precision DC characteristics. Recommended values

for C

are between 1nF and 10nF. Additional anti-alias-

ing filtering at the output can further reduce this auto-

correction noise.

Capacitive-Load Stability

The MAX4208/MAX4209 are capable of driving capaci-

tive loads up to 200pF. Applications needing higher

capacitive drive capability may use an isolation resistor

between OUT and the load to reduce ringing on the

output signal. However, this reduces the gain accuracy

due to the voltage drop across the isolation resistor.

GAIN

⎛

⎝

⎜

⎞

⎠

⎟

MAX4208/MAX4209

Ultra-Low Offset/Drift, Precision

Instrumentation Amplifiers with REF Buffer

14 ______________________________________________________________________________________

Power-Supply Bypass and Layout

Good layout technique optimizes performance by

decreasing the amount of stray capacitance at the

instrumentation amplifier’s gain-setting pins (OUT, FB,

and REF). Excess capacitance produces peaking in

the amplifier’s frequency response. To decrease stray

capacitance, minimize trace lengths by placing exter-

nal components as close as possible to the instrumen-

tation amplifier. Unshielded long traces at the inputs of

the instrumentation amplifier degrade the CMRR and

pick-up noise. This produces inaccurate output in high-

gain configurations. Use shielded or coax cables to

connect the inputs of the instrumentation amplifier.

Since the MAX4208/MAX4209 feature ultra-low input

offset voltage, board leakage and thermocouple effects

can easily introduce errors in the input offset voltage

readings when used with high-impedance signal

sources. Minimize board leakage current and thermo-

couple effects by thoroughly cleaning the board and

placing the matching components very close to each

other and with appropriate orientation. For best perfor-

mance, bypass each power supply to ground with a

separate 0.1µF capacitor.

For noisy digital environments, the use of multilayer

PCB with separate ground and power-supply planes is

recommended. Keep digital signals far away from the

sensitive analog inputs.

Refer to the MAX4208 or MAX4209 Evaluation Kit data

sheets for good layout examples.

Low-Side Current-Sense Amplifier

The use of indirect current-feedback architecture

makes the MAX4208/MAX4209 ideal for low-side cur-

rent-sensing applications, i.e., where the current in the

circuit ground needs to be measured by means of a

small sense resistor. In these situations, the input com-

mon-mode voltage is allowed to be at or even slightly

below ground (V

- 0.1V).

If the currents to be measured are bidirectional, con-

nect REFIN/MODE to V

/2 to get full dynamic range

for each direction. If the currents to be measured are

unidirectional, both REFIN/MODE and REF can be tied

to GND. However, VOL limitations can limit low-current

measurement. If currents need to be measured down to

0A, bias REFIN/MODE to a voltage above 0.2V to acti-

vate the internal buffer and to stay above amplifier VOL,

and measure both OUT and REF with a differential

input ADC.

Low-Voltage, High-Side

Current-Sense Amplifier

Power management is a critical area in high-perfor-

mance portable devices such as notebook computers.

Modern digital processors and ASICs are using smaller

transistor geometries to increase speed, reduce size,

and also lower their operating core voltages (typically

0.9V to 1.25V). The MAX4208/MAX4209 instrumentation

amplifiers can be used as a nearly zero voltage-drop,

current-sense amplifier (see Figure 5).

CM-MAX

3/4 V

1/2 V

1/4 V

OUT

( = GAIN x V

DIFF

+ V

REF

)

REF

= 1/2 V

CLASSIC THREE OP-AMP INA

CM-MAX

OUT

( = GAIN x V

DIFF

+ V

REF

)

REF

= 1/2 V

MAX4208/MAX4209

Figure 3. Limited Common Mode vs. Output Voltage of a

Three Op-Amp INA

Figure 4. Input Common Mode vs. Output Voltage of

MAX4208/MAX4209 Includes 0V (GND)

MAX4208/MAX4209

Ultra-Low Offset/Drift, Precision

Instrumentation Amplifiers with REF Buffer

______________________________________________________________________________________ 15

The ultra-low V

of the MAX4208/MAX4209 allows full-

scale V

SENSE

of only 10mV to 20mV for minimally inva-

sive current sensing using milliohm sense resistors to

get high accuracy. Previous methods used the internal

resistance of the inductor in the step-down DC-DC con-

verter to measure the current, but the accuracy was

only 20% to 30%. Using a full-scale V

SENSE

of 20mV, a

20µV max, V

error term is less than 0.1% and

MAX4209H gain error is 0.25% max at 100x, so the

total accuracy is greatly improved. The 0 to 2V output

of MAX4209H can be sent to an ADC for calculation.

The adjustable gain of MAX4208, can be set to a gain

of 250x using 1kΩ and 249kΩ resistors, to scale up a

lower 10mV V

SENSE

voltage to a larger 2.5V output volt-

age for wider dynamic range as needed.

ASIC

IN+

IN-

REF

REFIN/MODE

1V AT 10A

ADC

ANTI-ALIASING

FILTER

0.002Ω

SENSE

MAX4209H

OUT

+3.3V

SENSE

= 10A x 0.002Ω = 20mV

POWER IN R

SENSE

= 10A x 20mV = 200mW

OUT = G x 20mV = 100 x 20mV = 2V

Figure 5. MAX4208/MAX4209 Used as Precision Current-Sense Amplifiers for Notebook Computers with V

SENSE

of 20mV

IN-

IN+

REFIN/MODE

REF

MAX4208

OUT

REF

G = 1 +

BUFFER OUT =

Typical Application Circuit

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P17

MAX4208AUA+T

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Instrumentation Amplifiers Low Offset/Drift Instrumentation Amp

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MAX4208AUA+T

MAX4208AUA+T

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