REV. A

AD8225

–11–

A1

A2

C2

R2

–IN

Q2

C1

R1

+IN

Q1

3k

3k

15k

15k

A3

OUT

Figure 3. Simplified Schematic

THEORY OF OPERATION

The AD8225 is a monolithic, three op amp instrumentation

amplifier. Laser wafer trimming and proprietary circuit tech-

niques enable the AD8225 to boast the lowest output offset

voltage and drift of any currently available in amp (150

µV

RTI), as well as a higher common-mode voltage range.

Referring to Figure 3, the input buffers consist of super-beta

NPN transistors Q1 and Q2, and op amps A1 and A2. The

transistors are compensated so that the bias currents are

extremely low, typically 100 pA or less. As a result, current noise

is also low, at 50 fA/

√Hz. The unity gain input buffers drive a

gain-of-five difference amplifier. Because the 3 k

Ω and 15 kΩ

resistors are ratio matched, gain stability is better than 5 ppm/

°C

over the rated temperature range.

The AD8225 also has five times the gain bandwidth of a typical

in amp. This wider GBW results from compensation at a fixed

gain of 5, which can be one fifth of that required if the amplifier

were compensated for unity gain.

High frequency performance is also enhanced by the innovative

pinout of the AD8225. Since Pins 1 and 8 are uncommitted,

Pin 1 may be connected to Pin 4. Since Pin 4 is also ac com-

mon, the stray capacitance at Pins 2 and 3 is balanced.

8

7

6

5

NC

REF

AD8225

+IN

–IN

AC

GROUND

AC

GROUND

PIN 1 HAS NO INTERNAL CONNECTION

Figure 4. Pinout for Symmetrical Input Stray Capacitance

APPLICATIONS

Precision V-to-I Converter

When small analog voltages are transmitted across significant

distances, errors may develop due to ambient electrical noise,

stray capacitance, or series impedance effects. If the desired

voltage is converted to a current, however, the effects of ambient

noise are mitigated. All that is required is a voltage to current

conversion at the source, and an I-to-V conversion at the other

end to reverse the process.

Figure 5 illustrates how the AD8225 may be used as the trans-

mitter and receiver in a current loop system. The full-scale

output is 5 mA.

AD8225

2

3

5

6

47pF

9k

20

AD8225

2

3

5

6

8

GND OR

REF V

1k

FULL SCALE

CURRENT = 5mA

200mV

pk

FS

200mV

pk FS

=

=

OP27

Figure 5. Precision Voltage-to-Current Converter

As noted in Figure 5, an additional op amp and four resistors are

required to complete the converter. The precision gain of 5 in the

AD8225s, used in the transmit and receive sections, preserves

the integrity of the desired signal, while the high frequency

common-mode performance at the receiver rejects noise on the

transmission line. The reference of the receiver may be connected

to local ground or the reference pin of an A/D converter (ADC).

Figure 6 shows bench measurements of the input and output

voltages, and output current of the circuit of Figure 5. The

transmission media is 10 feet of insulated hook-up wire for the

current drive and return lines.

e

IN

e

OUT

CH 1 = 100mV, CH 2 = 100mV, CH 3 = 10mA,

H = 200 s

e

1

2

3