AD548 Analog Devices, AD548 Datasheet - Page 6

• Manufacturer Part Number: AD548
• Description: Precision/ Low Power BiFET Op Amp
• Manufacturer: Analog Devices

AD548
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 con-
dition. Figure 23 shows a simple current limiting scheme that
can be used. R
mum overload current is 1.0 mA (l00 k 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 doesn’t 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.
D/A CONVERTER OUTPUT BUFFER
The circuit in Figure 24 shows the AD548 and AD7545 12-bit
CMOS D/A converter in a unipolar binary configuration. V
will be equal to V
digital word. V
adjusting R
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.
Figure 22. Board Layout for Guarding Inputs
Figure 23. Input Protection of IV Converter
IN
. The AD548’s low input offset voltage, low drift
PROTECT
REF
REF
sets the full scale. Overall gain is trimmed by
attenuated by a factor depending on the
should be chosen such that the maxi-
OUT
–6–
That is:
R
the DAC. R
value of R
the offset error voltage at the amplifier’s output. An output am-
plifier 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 sig-
nal bandwidth and 1.8 V/ s typical slew rate. The 33 pF capaci-
tor across the feedback resistor optimizes the circuit’s response.
The oscilloscope photos in Figures 25 and 26 show small and
large signal outputs of the circuit in Figure 24. Upper traces
show the input signal V
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 s.
Figure 26. Response to 100 mV p-p Reference Square
Wave
Figure 25. Response to 20 V p-p Reference Square Wave
FB
is the feedback resistor for the op amp, which is internal to
Figure 24. AD548 Used as DAC Output Amplifier
O
is code dependent. This has the effect of changing
O
V
is the DAC’s R-2R ladder output resistance. The
OS
10
10
100
100
0%
0%
90
90
50mV
Output V
5V
IN
. Lower traces are the resulting output
200mV
20V
OS
Input 1
2µS
R
R
5µS
FB
O
REV. C