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.
PS024705-0405
P R E L I M I N A R Y
Analog-to-Digital Converter
ZILOG [ ZILOG, INC. ]
