REV. D

MAT04

–8–

Figure 5. Vector Summer

LOW NOISE, HIGH SPEED INSTRUMENTATION

AMPLIFIER

The circuit of Figure 6 is a very low noise, high speed amplifier,

ideal for use in precision transducer and professional audio

applications. The performance of the amplifier is summarized in

Table I. Figure 7 shows the input referred spot noise over the

0–25 kHz bandwidth to be flat at 1.2 nV/

√Hz. Figure 20 high-

lights the low 1/f noise corner at 2 Hz.

The circuit uses a high speed op amp, the OP17, preceded by

an input amplifier. This consists of a precision dual matched-

transistor, the MAT02, and a feedback V-to-I converter, the

MAT04. The arrangement of the MAT04 is known as a “linear-

ized cross quad” which performs the voltage-to-current conversion.

The OP17 acts as an overall nulling amplifier to complete the

feedback loop. Resistors R1, R2, and R3, R4 form voltage divid-

ers that attenuate the output voltage swing since the “cross

quad” arrangement has a limited input range. Biasing for the in-

put stage is set by Zener diode Z1. At low currents, the effective

zener voltage is about 3.3 V due to the soft knee characteristic of

the Zener diode. This results in a bias current of 530

µA per

side for the input stage. The gain of this amplifier with the val-

ues shown in Figure 6 is:

V

VR

OUT

IN

G

=

33000

Table I. Instrumentation Amplifier Characteristics

Input Noise

G = 1000

1.2 nV/

√Hz

Voltage Density

G = 100

3.6 nV/

√Hz

G = 10

30 nV/

√Hz

Bandwidth

G = 500

400 kHz

G = 100

1 MHz

G = 10

1.2 MHz

Slew Rate

40 V/

µs

Common-Mode Rejection G = 1000

130 dB

Distortion

G = 100

f = 20 Hz to 20 kHz

0.03%

Settling Time

G = 1000

10

µs

Power Consumption

350 mW