LOG112, 2112

8

SBOS246C

www.ti.com

FIGURE 7. Precision Current Inverter/Current Source.

1.5k

Ω

Photodiode

10nA to 1mA

10nA to 1mA

+5V

+5V

1k

Ω

+3.3V

+3.3V

1/2

OPA2335

1/2

OPA2335

BSH203

TLV271 or 1/2 OPA2335

Back Bias

LOG112

Pin 1 or Pin 14

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

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

LOG112 and LOG2112’s input offset voltage.

FREQUENCY COMPENSATION

Frequency compensation for the LOG112 is obtained by

connecting a capacitor between pins 5 and 14. Frequency

compensation for the LOG2112 is obtained by connecting a

capacitor between pins 2 and 5, or 15 and 12. The size of the

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

Typical Characteristic curves (Minimum Value of Compensa-

tion Capacitor). For any given application, the smallest value

of the capacitor which may be used is determined by the

but reduce the frequency response.

In an application, highest overall bandwidth can be achieved

appropriate values of compensation capacitors.

NEGATIVE INPUT CURRENTS

The LOG112 and LOG2112 function only with positive input

currents (conventional current flows into input current pins).

In situations where negative input currents are needed, the

circuits in Figures 6, 7, and 8 may be used.

FIGURE 5. Current Source with Offset Compensation.

10M

Ω

+25mV

100k

Ω

–2.5V

+2.5V

OPA335

100

Ω

I

I

V

LOGOUT

C

C

14

1

910

V–

LOG112

6

5

V+

8

V

REF

Chopper Op Amp

GND

FIGURE 6. Current Inverter/Current Source.

Q

B

National

LM394

I

IN

I

OUT

D

1

OPA703

D

2

Q

A

FREQUENCY RESPONSE

The frequency response curves seen in the Typical Charac-

small-signal AC current on one input.

The 3dB frequency response of the LOG112 and LOG2112 are

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 LOG112 and LOG2112 are

different for increasing 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 LOG112

and LOG2112.