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LOG102

7

SBOS211A

FIGURE 6. Current Inverter/Current Source.

10M

Ω

+25mV

100k

Ω

–2.5V

+2.5V

OPA335

100

Ω

I

I

V

LOGOUT

C

C

1G

Ω to 2.5kΩ

REF3025

14

5

1

910

5

V–

LOG102

6

2.5V

V+

V+

Chopper Om Amp

FIGURE 5. Current Source with Offset Compensation.

Figure 5 shows a low-level current source using a series

resistor. The low offset op-amp reduces the effect of the

LOG102’s input offset voltage.

FIGURE 7. Precision Current Inverter/Current Source.

Q

B

National

LM394

I

IN

I

OUT

D

1

OPA703

D

2

Q

A

1.5k

Ω

Photodiode

10nA to 1mA

10nA to 1mA

+5V

+5V

1.5k

Ω

+3.3V

+3.3V

1/2 OPA2335

1/2 OPA2335

OPA2335

1

2

BSH203

TLV271 or

Back Bias

LOG102

Pin 1 or Pin 14

NOTE: OPA2335 Available Q2 2002

NEGATIVE INPUT CURRENTS

The LOG102 will function only with positive input currents

(conventional current flow into pins 1 and 14). In situations

where negative input currents are needed, the circuits in

Figures 6, 7, 8, and 9, may be used.

VOLTAGE INPUTS

The LOG102 gives the best performance with current inputs.

Voltage inputs may be handled directly with series resistors,

but the dynamic input range is limited to approximately three

decades of input voltage by voltage noise and offsets. The

transfer function of equation (14) applies to this configura-

tion.

FREQUENCY RESPONSE

The 3dB frequency response of the LOG102 is a function of

the magnitude of the input current levels and of the value of

the frequency compensation capacitor. See Typical Charac-

teristic Curves for details.

The frequency response curves are shown for constant DC

The transient response of the LOG102 is different for in-

creasing and decreasing signals. This is due to the fact that

a log amp is a nonlinear gain element and has different gains

at different levels of input signals. Smaller input currents

require greater gain to maintain full dynamic range, and will

slow the frequency response of the LOG102.

FREQUENCY COMPENSATION

Frequency compensation for the LOG102 is obtained by

connecting a capacitor between pins 5 and 14. The size of

the capacitor is a function of the input currents, as shown in

the Typical Characteristic Curves (Minimum Value of Com-

pensation Capacitor). For any given application, the smallest

value of the capacitor which may be used is determined by

reduce the frequency response.

In an application, highest overall bandwidth can be achieved

appropriate values of compensation capacitors.

can be scaled by increasing or decreasing the resistor ratio

connected to pins 4 and 7. The gain, G, can be set according

to the following equation:

(2)