A

1

+

–

R

1

R

3

A

3

+

–

R

2

R

4

A

2

+

–

I2

I1

V

CM IN

COM

A

4

+

–

V

O4

–IN

4

+IN

4

V+

V

REF GND

I

2

I

1

LOG114

V

LOGOUT

REF

V

REF

A

5

–

+

V

O5

+IN

5

–IN

5

R

5

R

6

V–

R

REF

I

REF

200

200

1.25k

1.25k

Figure 7-1. Example of Setting IREF

It can be difficult to achieve an accurate IREF at lower current values (<20nA). Rather than choosing an IREF

value to be equal to ISIGNAL (min), higher accuracy can be achieved by selecting IREF to be in the center of the full

signal range as shown in Equation 3.

IREF=ISIGNAL min × ISIGNAL max

ISIGNAL min

(3)

For example, for a signal range of 1nA to 1mA, after plugging in the values into Equation 3, IREF = 1μA. It is

much easier and more precise (that is, DC accuracy, temperature stability, and lower noise) to establish a 1μA

DC current level than a 1nA level for the reference current.

The reference current may be derived from a voltage source with one or more resistors. When a single resistor is

used, the value may be large depending on IREF. If IREF is 10nA and 2.5V is used:

RREF=VSOURCEIREF=2.5V10nA=250MΩ

(4)

A voltage divider T-Network circuit can be used to reduce the value of the resistor, as shown in Figure 7-2. When

using this method, consider the possible errors caused by the amplifier input offset voltage. The input offset

voltage of amplifier A1 has a maximum value of 4mV in a ±5V supply system, and a maximum value of 7mV in a

5V supply system. Consider resistor temperature stability and noise contributions, as well.

A

1

+

–

V

OS

R

3

R

1

R

2

I

REF

5V

V

REF = 100mV

Figure 7-2. T-Network for Reference Current

VREF may be an external precision voltage reference, or the on-chip 2.5V voltage reference of the LOG114.

IREF can be derived from an external current source, such as that shown in Figure 7-3.

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LOG114

SBOS301B – MAY 2004 – REVISED MARCH 2025

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