Datasheet

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7 Using the EVM

CONVST

RESET

SN74AHC138

DC_CS

Using the EVM

The ADS7881EVM/ADS7891EVM serves as a reference design, prototype board and as test platform for

the software engineer to develop code.

As a reference design, the ADS7881EVM/ADS7891EVM contains the essential circuitry to showcase the

analog-to-digital converter. This essential circuitry includes the input amplifier, reference circuit, and

buffers. The EVM analog input circuit is optimized for 1 MHz sine wave, therefore users may need to

adjust the resistor and capacitor values of the A/D input RC circuit. In ac type applications where signal

distortion is concern, polypropylene capacitors should be used in the signal path. In applications were the

input is multiplexed, the A/D input resistor and capacitor may need to adjusted or possibly removed

altogether.

As a prototype board, the buffer circuit consists of footprint is a standard 8 pin SOIC and resistor pads for

inverting and non-inverting configurations. The ADS7881EVM/ADS7891EVM can be used to evaluate both

dual and single supply amplifiers. The EVM comes installed with a dual supply amplifier as it allows the

user to take advantage of the full input voltage range of the converter. For applications that require signal

supply operation and smaller input voltage range, the THS4031 can be replaced with the single supply

amplifier like OPA300. Pad jumper SJP7 should be shorted between pads 1 and 2, as it shorts the minus

supply pin of the amplifier to ground. Positive supply voltage can be applied via test point TP12 or

connector J1 pin 1.

As a software test platform, connectors P1, P2, P3 plug into the parallel interface connectors of the 5-6K

interface card. The 5-6K interface card sits on the C5000 and C6000 Digital Signal Processor starter kit

(DSK). The ADS7881EVM/ADS7891EVM is then mapped into the processor's memory space. This card

also provides an area for signal conditioning. This area can be used to install application circuit(s) for

digitization by the ADS7881 and/or ADS7891 analog-to-digital converter. Refer to the 5-6K interface card

user's guide (SLAU104 ) for more information.

For the software engineer the ADS7881EVM/ADS7891EVM provides a simple platform for interfacing to

the converter. The EVM provides standard 0.1" headers and sockets to wire into prototype boards. The

user need only provide in 3 address lines (A2, A1, A0) and address valid line( DC_CS) to connector P2, as

shown in Figure 2 . To choose which address combinations will generate RD, CONVST, and RESET set

jumpers as shown in Table 4 . Recall chip select ( CS) signal is not memory mapped or tied to P2, therefore

it must be controlled via general purpose pin or shorted to ground at J3 pin 1. If address decoding is not

required, the EVM provides direct access to converter data bus via P3 and control via J3.

Figure 2. Decoding Control Signals Using the Address Bus

ADS7881/ADS7891EVMSLAU150 – December 2004 7