Z8F082ASH020SC datasheet(134/244 Pages) ZILOG

134 / 244 page

PS024705-0405

P R E L I M I N A R Y

Analog-to-Digital Converter

Z8 Encore! XP® F08xA Series

Product Specification

116

Transimpedance Amplifier

The transimpedance amplifier is a standard operational amplifier designed for current

measurements. Each of the three ports of the amplifier is accessible from the package pins.

The inverting input is commonly used to connect to the current source. The output node

connects an external feedback network to the inverting input. The non-inverting output is

required to apply a non-zero bias point. In a standard, single-supply system, this bias point

must be substantially above ground to measure positive input currents. The non-inverting

input may also be used for offset correction.

The transimpedance amplifier contains only one pin configuration: ANA0 is the output/

feedback node, ANA1 is the inverting input and ANA2 is the non-inverting input.

To use the transimpedance amplifier, it must be enabled in the Power Control Register 0

(PWRCTL0)

. The default state of the transimpedance amplifier is OFF. To use the transim-

pedance amplifier, the TRAM bit must be cleared, turning it ON (“Power Control Register

0 (PWRCTL0)” on page 30

). When making normal ADC measurements on ANA0 (not tran-

simpedance measurements), the TRAM bit must be OFF. Turning the TRAM bit ON inter-

feres with normal ADC measurements. Finally, this bit enables the amplifier even in

STOP mode. If the amplifier is not required in STOP mode, disable it. Failing to perform

this results in STOP mode currents greater than specified.

As with other ADC measurements, any pins used for analog purposes must be configured

as such in the GPIO registers (see “Port A–D Alternate Function Sub-Registers” on

page 39

Standard transimpedance measurements are made on ANA0, as selected by the

ANAIN[3:0] bits of ADC Control Register 0. It is also possible to make single-ended mea-

surements on ANA1 and ANA2 while the amplifier is enabled, which is often useful for

determining offset conditions.

The BUFFMODE[2:0] bits of ADC Control/Status Register 1 must also be configured for sin-

gle-ended, unity-gain buffered operation. Using the transimpedance amplifier in an unbuf-

fered or differential mode is not recommended.

When either input is overdriven, the amplifier output saturates at the positive or negative

supply voltage. No instability results.

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