AD8253ARMZ-R7 datasheet(17/24 Pages) AD

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AD8253

Rev. 0 | Page 17 of 24

Table 5. Truth Table Logic Levels for Transparent Gain Mode

Gain

−VS

Low

−VS

Low

High

−VS

High

Low

100

−VS

High

1000

Latched Gain Mode

Some applications have multiple programmable devices such

as multiplexers or other programmable gain instrumentation

amplifiers on the same PCB. In such cases, devices can share a

data bus. The gain of the AD8253 can be set using WR as a latch,

allowing other devices to share A0 and A1.

shows a

schematic using this method, known as latched gain mode. The

AD8253 is in this mode when

Figure 53

WR is held at logic high or logic

low, typically 5 V and 0 V, respectively. The voltages on A0 and A1

are read on the downward edge of the WR signal as it transitions

from logic high to logic low. This latches in the logic levels on

A0 and A1, resulting in a gain change. See the truth table listing

for more on these gain changes.

Table 6

+15V

–15V

+IN

–IN

10μF0.1µF

DGND

REF

AD8253

+5V

G = PREVIOUS

STATE

G = 1000

–

NOTE:

1. ON THE DOWNWARD EDGE OF WR, AS IT TRANSITIONS

FROM LOGIC HIGH TO LOGIC LOW, THE VOLTAGES ON A0

AND A1 ARE READ AND LATCHED IN, RESULTING IN A

GAIN CHANGE. IN THIS EXAMPLE, THE GAIN SWITCHES TO G = 1000.

Figure 53. Latched Gain Mode, G = 1000

Table 6. Truth Table Logic Levels for Latched Gain Mode

Gain

High to Low

Low

Change to 1

High to Low

Low

High

Change to 10

High to Low

High

Low

Change to 100

High to Low

High

Change to 1000

Low to Low

No change

Low to High

No change

High to High

No change

1 X = don’t care.

On power-up, the AD8253 defaults to a gain of 1 when in

latched gain mode. In contrast, if the AD8253 is configured in

transparent gain mode, it starts at the gain indicated by the

voltage levels on A0 and A1 on power-up.

Timing for Latched Gain Mode

In latched gain mode, logic levels at A0 and A1 must be held for

a minimum setup time, tSU, before the downward edge of WR

latches in the gain. Similarly, they must be held for a minimum

hold time, tHD, after the downward edge of WR to ensure that

the gain is latched in correctly. After tHD, A0 and A1 may change

logic levels, but the gain does not change until the next downward

edge of WR. The minimum duration that WR can be held high

is t WR-HIGH, and t WR-LOW is the minimum duration that WR can

be held low. Digital timing specifications are listed in

The time required for a gain change is dominated by the settling

time of the amplifier. A timing diagram is shown in

Table 2.

Figure 54

When sharing a data bus with other devices, logic levels applied

to those devices can potentially feed through to the output of

the AD8253. Feedthrough can be minimized by decreasing the

edge rate of the logic signals. Furthermore, careful layout of the

PCB also reduces coupling between the digital and analog

portions of the board.

A0, A1

tSU

tHD

tWR-HIGH

tWR-LOW

Figure 54. Timing Diagram for Latched Gain Mode

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