AD8556ACPZ-R2 datasheet(24/28 Pages) AD

24 / 28 page

Data Sheet

AD8556

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 only connected to the second gain stage

output via RF. During read mode, however, the FILT/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 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 out the shift

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 eight bits long,

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

For example, to read the second stage gain, this code is used:

1000 0000 0001 11 00 10 0000 0000 0111 1111 1110. Because

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 pulses at DIGIN return the 3-bit

value at FILT/DIGOUT, the seventh pulse returns 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. 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 AD8556 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, which is

much lower than 1.5 V; therefore, 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; therefore, 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.

Therefore, the OTP cell could output Logic 0 or 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 specific 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 Logic 0 instead of

the expected Logic 1 from a blown fuse. Fuses blown correctly

would still output Logic 1. In this way, fuses blown incorrectly

can be detected. Another specific code would return the sense

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

be permanently programmed to the low value. When the AD8556

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

The low sense current code is:

1000 0000 0001 00 00 10 XXXX XXX1 0111 1111 1110.

The normal (high) sense current code is:

1000 0000 0001 00 00 10 XXXX XXX0 0111 1111 1110.

Programming Procedure

For reliable fuse programming, it is imperative to follow the

programming procedure requirements, especially the proper

supply voltage during programming.

1. When programming the AD8556, the temperature of the

device must be between 10°C and 40°C.

2. Set VDD and VSS to the desired values in the application.

Use simulation mode to test and determine the desired

codes for the second stage gain, first stage gain, and output

offset. The nominal values for these parameters are shown

in Table 6, Table 7, Equation 2, and Equation 3; use the

codes corresponding to these values as a starting point.

However, because actual parameter values for given codes

vary from device to device, some fine tuning is necessary

for the best possible accuracy.

One way to choose these values is to set the output offset

to an approximate value, such as Code 128 for midsupply,

to allow the required gain to be determined. Then set the

second stage gain so the minimum first stage gain (Code 0)

gives a lower gain than required, and the maximum first

stage gain (Code 127) gives a higher gain than required.

After choosing the second stage gain, the first stage gain

can be chosen to fine tune the total gain. Finally, the output

offset can be adjusted to give the desired value. After

determining the desired codes for second stage gain,

first stage gain, and output offset, the device is ready for

permanent programming.

Rev. B | Page 23 of 27

AD8556ACPZ-R2 数据表(PDF) 24 Page - Analog Devices

AD8556ACPZ-R2 数据表(HTML) 24 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

数据表下载