AD8557ACPZ-R2 datasheet(18/24 Pages) AD

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AD8557

Rev. C | Page 18 of 24

Simulation Mode

The simulation mode allows any parameter to be temporarily

changed. These changes are retained until the simulated value is

reprogrammed, the power is removed, or the master fuse is

blown. Parameters are simulated by setting Field 1 to 01,

selecting the desired parameter in Field 2, and selecting the

desired value for the parameter in Field 4. Note that a value of

11 for Field 2 is ignored during the simulation mode. Examples

of temporary settings follow:



Setting the second stage gain code (Parameter 00) to 011

and the second stage gain to 40 produces:

1000 0000 0001 01 00 10 0000 0011 0111 1111 1110



Setting the first stage gain code (Parameter 01) to 000 1011

and the first stage gain to 4.166 produces:

1000 0000 0001 01 01 10 0000 1011 0111 1111 1110

A first stage gain of 2.954 with a second stage gain of 40

gives a total gain of 118.16. This gain has a maximum

tolerance of 2.5%.



Set the output offset code (Parameter 10) to 0100 0000

and the output offset to 1.260 V when VDD = 5 V and

VSS = 0 V. This output offset has a maximum tolerance

of 0.8%:

1000 0000 0001 01 10 10 0100 0000 0111 1111 1110

Programming Mode

Intact fuses give a bit value of 0. Bits with a desired value of 1

need to have the associated fuse blown. Because a relatively

large current is needed to blow a fuse, only one fuse can be

reliably blown at a time. Thus, a given parameter value may

need several 38-bit words to allow reliable programming.

A 5.75 V (±0.25 V) supply is required when blowing fuses to

minimize the on resistance of the internal MOS switches that

blow the fuse. The power supply voltage must not exceed the

absolute maximum rating and must be able to deliver 250 mA

of current.

At least 10 μF (tantalum type) of decoupling capacitance is

needed across the power pins of the device during program-

ming. The capacitance can be on the programming apparatus as

long as it is within 2 inches of the device being programmed.

An additional 0.1 μF (ceramic type) in parallel with the 10 μF is

recommended within ½ inch of the device being programmed.

A minimum period of 1 ms should be allowed for each fuse to

blow. There is no need to measure the supply current during

programming.

The best way to verify correct programming is to use the read

mode to read back the programmed values. Then, remeasure

the gain and offset to verify these values. Programmed fuses

have no effect on the gain and output offset until the master

fuse is blown. After blowing the master fuse, the gain and

output offset are determined solely by the blown fuses, and the

simulation mode is permanently deactivated.

Parameters are programmed by setting Field 1 to 10, selecting

the desired parameter in Field 2, and selecting a single bit with

the value 1 in Field 4.

As an example, suppose the user wants to permanently set the

second stage gain to 40. Parameter 00 needs to have the value

0000 0011 assigned. Two bits have the value 1, so two fuses need

to be blown. Because only one fuse can be blown at a time, this

code can be used to blow one fuse:

1000 0000 0001 10 00 10 0000 0010 0111 1111 1110

The MOS switch that blows the fuse closes when the complete

packet is recognized, and opens when the start-of-packet,

dummy, or end-of-packet fields are no longer valid. After 1 ms,

this second code is entered to blow the second fuse:

1000 0000 0001 10 00 10 0000 0001 0111 1111 1110

To permanently set the first stage gain to a nominal value of

2.954, Parameter 01 needs to have the value 000 1011 assigned.

Three fuses need to be blown, and the following codes are used,

with a 1 ms delay after each code:

1000 0000 0001 10 01 10 0000 1000 0111 1111 1110

1000 0000 0001 10 01 10 0000 0010 0111 1111 1110

1000 0000 0001 10 01 10 0000 0001 0111 1111 1110

To permanently set the output offset to a nominal value of

1.260 V when VDD = 5 V and VSS = 0 V, Parameter 10 needs

to have the value 0100 0000 assigned. If one fuse needs to be

blown, use the following code:

1000 0000 0001 10 10 10 0100 0000 0111 1111 1110

Finally, to blow the master fuse to deactivate the simulation

mode and prevent further programming, use code:

1000 0000 0001 10 11 10 0000 0001 0111 1111 1110

There are a total of 20 programmable fuses. Because each fuse

requires 1 ms to blow, and each serial word can be loaded in

2.3 ms, the maximum time needed to program the fuses can be

as low as 66 ms.

Read Mode

The values stored by the polysilicon fuses can be sent to the

DIGOUT pin to verify correct programming. Normally, the

DIGOUT pin is only connected to the second gain stage output.

During read mode, however, the DIGOUT pin is also connected

to the output of a shift register to allow the polysilicon fuse

contents to be read. Because VOUT is a buffered version of

DIGOUT, VOUT also outputs a digital signal during read mode.

Read mode is entered by setting Field 1 to 11 and selecting the

desired parameter in Field 2. Field 4 is ignored. The parameter

value, stored in the polysilicon fuses, is loaded into an internal

shift register, and the MSB of the shift register is connected to

the DIGOUT pin. Pulses at DIGIN shift out the shift register

contents to the DIGOUT pin, allowing the 8-bit parameter

value to be read after seven additional pulses; shifting occurs on

the falling edge of DIGIN. An eighth pulse at DIGIN disconnects

DIGOUT from the shift register and terminates the read mode.

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