AD548BH Просмотр технического описания (PDF) - Analog Devices

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AD548BH

Precision, Low Power / High Performance / BiFET Operational Amplifiers

Analog Devices 

AD548

Figure 2. Board Layout for Guarding Inputs

INPUT PROTECTION

The AD548 is guaranteed to withstand input voltages equal to

the power supply potential. Exceeding the negative supply volt-

age on either input will forward bias the substrate junction of

the chip. The induced current may destroy the amplifier due to

excess heat.

Input protection is required in applications such as a flame

detector in a gas chromatograph, where a very high potential

may be applied to the input terminals during a sensor fault

condition. Figure 3 shows a simple current limiting scheme that

can be used. RPROTECT should be chosen such that the maxi-

mum overload current is 1.0 mA (l00 kΩ for a 100 V overload,

for example).

Exceeding the negative common-mode range on either input

terminal causes a phase reversal at the output, forcing the

amplifier output to the corresponding high or low state. Exceed-

ing the negative common-mode on both inputs simultaneously

forces the output high. Exceeding the positive common-mode

range on a single input does not cause a phase reversal, but if

both inputs exceed the limit the output will be forced high. In

all cases, normal amplifier operation is resumed when input

voltages are brought back within the common-mode range.

Figure 4. AD548 Used as DAC Output Amplifier

That is:

VOS

Output

= VOS

Input

1+

RFB

RO





RFB is the feedback resistor for the op amp, which is internal to

the DAC. RO is the DAC’s R-2R ladder output resistance. The

value of RO is code dependent. This has the effect of changing

the offset error voltage at the amplifier’s output. An output

amplifier with a sub millivolt input offset voltage is needed to

preserve the linearity of the DAC’s transfer function.

The AD548 in this configuration provides a 700 kHz small

signal bandwidth and 1.8 V/µs typical slew rate. The 33 pF

capacitor across the feedback resistor optimizes the circuit’s

response. The oscilloscope charts in Figures 5 and 6 show small

and large signal outputs of the circuit in Figure 4. Upper traces

show the input signal VIN. Lower traces are the resulting output

voltage with the DAC’s digital input set to all 1s. The AD548

settles to ± 0.01% for a 20 V input step in 14 µs.

5V

100

90

20V

5µS

10

0%

Figure 5. Response to ±20 V p-p Reference Square Wave

Figure 3. Input Protection of IV Converter

D/A CONVERTER OUTPUT BUFFER

The circuit in Figure 4 shows the AD548 and AD7545 12-bit

CMOS D/A converter in a unipolar binary configuration. VOUT

will be equal to VREF attenuated by a factor depending on the

digital word. VREF sets the full scale. Overall gain is trimmed by

adjusting RIN. The AD548’s low input offset voltage, low drift,

and clean dynamics make it an attractive low power output buffer.

The input offset voltage of the AD548 output amplifier results

in an output error voltage. This error voltage equals the input

offset voltage of the op amp times the noise gain of the amplifier.

50mV

100

90

200mV

2µS

10

0%

Figure 6. Response to ±100 mV p-p Reference Square Wave

–8–

REV. D

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