LOG112: LOG112 about the reference circuit

Hi Mickey,

Here are answers to your LOG112 questions:

Q1: If I recommend LOG112 to the customer, which reference circuit does the customer use for his design?

I would suggest the circuit on page 1 of the LOG112 datasheet. It is quite complete for most applications. Do note the direction of the current flow into the LOG112 I1 input. If the input current has the opposite polarity, then one of the precision current inverter circuits shown in Figures 6 through 8 would be needed.

Q2: For LOG112, What are the voltage ranges for Vlogout and Vo3?

The VLOGOUT voltage range will depend on the I1 input current, and the I2 reference current. They will result in a VLOGOUT voltage that conforms with equation, VLOGOUT = (0.5V)LOG (I1/I2).

For example, if input current I1 = 100 pA and reference current I2 = 1 uA,

VLOGOUT = 0.5 V LOG( 1e-10 A/ 1e-6 A) = -2.00 V

if input current I1 = 1 nA and reference current I2 = 1 uA,

VLOGOUT = 0.5 V LOG( 1e-9 A/ 1e-6 A) = -1.50 V

The VLOGOUT output voltage can be gained up using the A3 gain stage. When this is done the VO3 output is determined from, 

VO3 = K (0.5V)LOG (I1/I2)        where: K = 1 + R2/R1 for a non-inverting gain configuration, or K = -R2/R1 for an inverting configuration

A3 can be arranged as either an inverting or non-inverting gain stage, but be sure that the gain isn't set too high such that the output saturates on peaks.

Regards, Thomas

Precision Amplifiers Applications Engineering