AD8555ACPZ-R2 datasheet(24/29 Pages) AD

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AD8555

Data Sheet

After the second stage gain, first stage gain, and output offset

have been programmed, DAT_SUM should be computed and

the parity bit should be set equal to DAT_SUM. If DAT_SUM is

0, the parity fuse should not be blown in order for the PFUSE

signal to be 0. If DAT_SUM is 1, the parity fuse should be

blown to set the PFUSE signal to 1. The code to blow the parity

fuse is 1000 0000 0001 10 11 10 0000 0100 0111 1111 1110.

After setting the parity bit, the master fuse can be blown to

prevent further programming, using the code 1000 0000 0001

10 11 10 0000 0001 0111 1111 1110.

Signal PAR_SUM is the output of the 2-input exclusive-OR

gate (Cell EOR2). After the master fuse has been blown,

PARITY_ERROR is set to PAR_SUM. As mentioned earlier,

the AD8555 behaves as a programmed amplifier when

PARITY_ERROR = 0 (no parity error). On the other hand,

VOUT is connected to VSS when a parity error has been

detected, i.e., when PARITY_ERROR = 1.

Read Mode

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

FILT/DIGOUT pin to verify correct programming. Normally,

the FILT/DIGOUT pin is connected to only the second gain

stage output via RF. During read mode, however, the

FILT/DIGOUT pin is also connected to the output of a shift

VOUT is a buffered version of FILT/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 FILT/DIGOUT pin. Pulses at DIGIN shift the shift register

contents out to the FILT/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 FILT/DIGOUT from the shift register and

terminates the read mode. If a parameter value is less than 8 bits

long, the MSBs of the shift register are padded with 0s.

For example, to read the second stage gain, the code 1000 0000

0001 11 00 10 0000 0000 0111 1111 1110 can be used. Since the

second stage gain parameter value is only three bits long, the

FILT/DIGOUT pin has a value of 0 when this code is entered

and remains 0 during four additional pulses at DIGIN. The

fifth, sixth, and seventh pulse at DIGIN returns the 3-bit value

at FILT/DIGOUT, the seventh pulse returning the LSB. An

eighth pulse at DIGIN terminates the read mode.

Sense Current

A sense current is sent across each polysilicon fuse to determine

whether it has been blown or not. When the voltage across the

fuse is less than approximately 1.5 V, the fuse is considered not

blown and Logic 0 is output from the OTP cell. When the

voltage across the fuse is greater than approximately 1.5 V, the

fuse is considered blown and Logic 1 is output.

When the AD8555 is manufactured, all fuses have a low

resistance. When a sense current is sent through the fuse, a

voltage less than 0.1 V is developed across the fuse. This is much

lower than 1.5 V, so Logic 0 is output from the OTP cell. When

a fuse is electrically blown, it should have a very high resistance.

When the sense current is applied to the blown fuse, the voltage

across the fuse should be larger than 1.5 V, so Logic 1 is output

from the OTP cell.

It is theoretically possible (though very unlikely) for a fuse

to be incompletely blown during programming, assuming the

required conditions are met. In this situation, the fuse could

have a medium resistance (neither low nor high), and a voltage

of approximately 1.5 V could be developed across the fuse. Thus,

the OTP cell could sometimes output Logic 0 or a Logic 1,

depending on temperature, supply voltage, and other variables.

To detect this undesirable situation, the sense current can be

lowered by a factor of 4 using a special code. The voltage developed

across the fuse would then change from 1.5 V to 0.38 V, and the

output of the OTP would be a Logic 0 instead of the Logic 1

expected from a blown fuse. Correctly blown fuses would still

output a Logic 1. In this way, incorrectly blown fuses can be

detected. Another special code would return the sense current

to the normal (larger) value. The sense current cannot be

permanently programmed to the low value. When the AD8555

is powered up, the sense current defaults to the high value.

The code to use the low sense current is 1000 0000 0001 00 00

10 XXXX XXX1 0111 1111 1110.

The code to use the normal (high) sense current is 1000 0000

0001 00 00 10 XXXX XXX0 0111 1111 1110.

Rev. B | Page 24 of 29

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