AD7841ASZ datasheet(9/13 Pages) AD

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AD7841

–9–

Power-On with

CLR Low

The output stage of the AD7841 has been designed to allow

output stability during power-on. If

CLR is kept low during

power-on, then just after power is applied to the AD7841, the

situation is as depicted in Figure 5. G1, G4 and G6 are open

while G2, G3 and G5 are closed.

DUTGND

VOUT

14k

DAC

Figure 5. Output Stage with VDD < 7 V or VSS > –3 V; CLR

Low

VOUT is kept within a few hundred millivolts of DUTGND via

G5 and a 14 k

Ω resistor. This thin-film resistor is connected in

parallel with the gain resistors of the output amplifier. The

output amplifier is connected as a unity gain buffer via G3, and

the DUTGND voltage is applied to the buffer input via G2. The

amplifier’s output is thus at the same voltage as the DUTGND

pin. The output stage remains configured as in Figure 5 until

the voltage at VDD exceeds 7 V and VSS is more negative than

–3 V. By now the output amplifier has enough headroom to

handle signals at its input and has also had time to settle. The

internal power-on circuitry opens G3 and G5 and closes G4 and

G6. This situation is shown in Figure 6. Now the output ampli-

fier is configured in its noise gain configuration via G4 and G6.

The DUTGND voltage is still connected to the noninverting

input via G2 and this voltage appears at VOUT.

DUTGND

VOUT

14k

DAC

Figure 6. Output Stage with VDD > 7 V and VSS < –3 V; CLR

Low

VOUT has been disconnected from the DUTGND pin by the

opening of G5, but will track the voltage present at DUTGND

via the configuration shown in Figure 6.

When

CLR is taken back high, the output stage is configured as

shown in Figure 7. The internal control logic closes G1 and

opens G2. The output amr})fier is connected in a noninverting

gain-of-two configuration. The voltage that appears on the VOUT

pins is determined by the data present in the DAC registers.

DUTGND

VOUT

14k

DAC

Figure 7. Output Stage After

CLR Is Taken High

Power-On with

CLR High

CLR is high on the application of power to the device, the

output stages of the AD7841 are configured as in Figure 8 while

VDD is less than 7 V and VSS is more positive than –3 V. G1 is

closed and G2 is open, thereby connecting the output of the

DAC to the input of its output amplifier. G3 and G5 are closed

while G4 and G6 are open, thus connecting the output amplifier as

a unity gain buffer. VOUT is connected to DUTGND via G5

through a 14 k

Ω resistor until V

DD exceeds 7 V and VSS is more

negative than –3 V.

DUTGND

VOUT

14k

DAC

Figure 8. Output Stage Powering Up with

CLR High

While VDD < 7 V or VSS > –3 V

When the difference between the supply voltages reaches 10 V,

the internal power-on circuitry opens G3 and G5 and closes G4

and G6 configuring the output stage as shown in Figure 9.

DUTGND

VOUT

14k

DAC

Figure 9. Output Stage Powering Up with

CLR High

When VDD > 7 V and VSS < –3 V

REV. B

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