LOG114AIRGVR datasheet(16/30 Pages) BURR-BROWN

16 / 30 page

LOG114

SBOS301A − MAY 2004 − REVISED MARCH 2007

www.ti.com

SETTING THE REFERENCE CURRENT

When the LOG114 is used to compute logarithms, ei-

ther I1 or I2 can be held constant to become the refer-

ence current to which the other is compared.

If IREF is set to the lowest current in the span of the signal

current (as shown in the front page figure), VLOGOUT will

range from:

LOGOUT + 0.375

log

1 min

1 max signal

^ 0V

to some maximum value:

LOGOUT + 0.375

log

1 min

1 max signal

While convenient, this approach does not usually result

in best performance, because I1 min accuracy is difficult

to achieve, particularly if it is < 20nA.

A better way to achieve higher accuracy is to choose

IREF to be in the center of the full signal range. For

example, for a signal range of 1nA to 1mA, it is better

to use this approach:

REF

+ I

SIGNAL min

1mA 1nA

+ 1mAdc

than it is to set IREF = 1nA. It is much easier and more

precise (that is, dc accuracy, temperature stability, and

lower noise) to establish a 1mA 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 resis-

tor is used, the value may be large depending on IREF.

If IREF is 10nA and +2.5V is used:

RREF = 2.5V/10nA = 250MΩ

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

resistor, as shown in Figure 5. When using this method,

one must 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 sup-

ply system, and a maximum value of 7mV in a +5V sup-

ply system. Resistor temperature stability and noise

contributions should also be considered.

+5V

REF =100mV

3 >> R2

REF

−

Figure 5. 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 6.

2N2905

REF

2N2905

+15V

−15V

REF =

REF

3.6k

Ω

IN834

Figure 6. Temperature-Compensated Current Source.

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