Analog And Digital Loop-Back Test Modes - Analog Devices AD9865 Manual

calibration and filter tuning routine is initiated upon RESET
returning high. To ensure sufficient power-on time of the
various functional blocks, RESET returning high should occur
no less than 10 ms upon power-up. If a digital reset signal from
a microprocessor reset circuit (such as ADM1818) is not
available, a simple R-C network referenced to DVDD can be
used to hold RESET low for approximately 10 ms upon power-
up.

ANALOG AND DIGITAL LOOP-BACK TEST MODES

The AD9865 features analog and digital loop-back capabilities
that can assist in system debug and final test. Analog loop-back
routes the digital output of the ADC back into the Tx data path
prior to the interpolation filters such that the Rx input signal
can be monitored at the output of the TxDAC or IAMP. As a
result, the analog loop-back feature can be used for a half- or
full-duplex interface, to allow testing of the functionality of the
entire IC (excluding the digital data interface).
For example, the user can configure the AD9865 with similar
settings as the target system, inject an input signal (sinusoidal
waveform) into the Rx input, and monitor the quality of the
reconstructed output from the TxDAC or IAMP to ensure a
minimum level of performance. In this test, the user can
exercise the RxPGA as well as validate the attenuation char-
acteristics of the RxLPF. Note that the RxPGA gain setting
should be selected such that the input does not result in clipping
of the ADC.
Digital loop-back can be used to test the full-duplex digital
interface of the AD9865. In this test, data appearing on the
Tx[5:0] port is routed back to the Rx[5:0] port, thereby
confirming proper bus operation. The Rx port can also be
three-stated for half- and full-duplex interfaces.
Table 26. SPI Registers for Test Modes
Address (Hex)
0x0D
Rev. A | Page 43 of 48
AD9865
Bit Description
(7) Analog loop-back
(6) Digital loop-back
(5) Rx port three-state