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LOG101

6

SBOS242B

FIGURE 6. Current Inverter/Current Source.

10M

Ω

+25mV

100k

Ω

–2.5V

+2.5V

OPA335

100

Ω

I

I

V

OUT

C

C

1G

Ω to 2.5kΩ

REF3025

8

3

1

56

V–

LOG101

4

3

2.5V

V+

V+

Chopper Op Amp

GND

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

LOG101’s input offset voltage.

FREQUENCY RESPONSE

The frequency response curve seen in the Typical Charac-

signal AC current on one input.

The 3dB frequency response of the LOG101 is a function of

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

frequency compensation capacitor. See Typical Characteristic

Curve “3dB Frequency Response” for details.

The transient response of the LOG101 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

Q

B

National

LM394

I

IN

I

OUT

D

1

OPA703

D

2

Q

A

2N2905

I

REF

R

REF

2N2905

+15V

–15V

I

6V

R

REF

3.6k

Ω

6V

IN834

FIGURE 3. Temperature Compensated Current Source.

SETTING THE REFERENCE CURRENT

which the other is compared.

shown in Figure 3), or it may be derived from a voltage

source with one or more resistors. When a single resistor is

10nA and +2.5V is used:

A

1

+

R

2

R

1

+5V

R

3

V

R

I

REF

–

V

OS

1

FIGURE 4. T Network for Reference Current.

A voltage divider may be used to reduce the value of the

resistor, as shown in Figure 4. When using this method, one

must consider the possible errors caused by the amplifier’s

value of 100mV.

at different levels of input signals. Smaller input currents

require greater gains to maintain full dynamic range, and will

slow the frequency response of the LOG101.

FREQUENCY COMPENSATION

Frequency compensation for the LOG101 is obtained by

connecting a capacitor between pins 3 and 8. The size of the

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

Typical Characteristic Curves (Minimum Value of Compen-

sation 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.

NEGATIVE INPUT CURRENTS

The LOG101 will function only with positive input currents

(conventional current flows into pins 1 and 8). In situations

where negative input currents are needed, the circuits in

Figures 6, 7, and 8 may be used.

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