Datasheet
OP AMP: FREQUENCY RESPONSE VERSUS ANALOG MUX
SR(V/ s)=2 f V (1 10 )m p ´ ´
OP
-6
(4)
Example:
PGA112 , , PGA113
PGA116 , PGA117
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............................................................................................................................................ SBOS424B – MARCH 2008 – REVISED SEPTEMBER 2008
GAIN
The analog input MUX provides two input channels
Table 8 documents how small-signal bandwidth and for the PGA112/PGA113 and 10 input channels for
slew rate change correspond to changes in PGA the PGA116/PGA117. The MUX switches are
gain. designed to be break-before-make and thereby
eliminate any concerns about shorting the two input
Full power bandwidth (that is, the highest frequency
signal sources together.
that a sine wave can pass through the PGA for a
given gain) is related to slew rate by Equation 4 : Four internal MUX CAL channels are included in the
analog MUX for ease of system calibration. These
CAL channels allow ADC gain and offset errors to be
calibrated out. This calibration does not remove the
Where:
offset and gain errors of the PGA for gains greater
SR = Slew rate in V/ µ s
than 1, but most systems should see a significant
f = Frequency in Hz
increase in the ADC accuracy. In addition, these CAL
V
OP
= Output peak voltage in volts
channels can be used by the ADC to read the
minimum and maximum possible voltages from the
PGA. With these minimum and maximum levels
known, the system architecture can be designed to
For G = 8, then SR = 10.6V/ µ s (slew rate rise is
indicate an out-of-range condition on the measured
minimum slew rate).
analog input signals if these levels are ever
For a 5V system, choose 0.1V < V
OUT
< 4.9V or
measured.
V
OUTPP
= 4.8V or V
OUTP
= 2.4V.
To use the CAL channels, V
CAL
/CH0 must be
SR (V/ µ s) = 2 π f × V
OP
(1 × 10
– 6
).
permanently connected to the system ADC reference.
10.6 = 2 π f (2.4) (1 × 10
– 6
) → f = 702.9kHz
There is a typical 100k Ω load from V
CAL
/CH0 to
This example shows that a G = 8 configuration
ground. Table 9 illustrates how to use the CAL
can produce a 4.8V
PP
sine wave with frequency
channels with V
REF
= ground. Table 10 describes how
up to 702.9kHz. This computation only shows the
to use the CAL channels with V
REF
= AV
DD
/2. The
theoretical upper limit of frequency for this
V
REF
pin must be connected to a source that is
example, but does not indicate the distortion of
low-impedance for both dc and ac in order to
the sine wave. The acceptable distortion depends
maintain gain and nonlinearity accuracy. Worst-case
on the specific application. As a general
current demand on the V
REF
pin occurs when G = 1
guideline, maintain two to three times the
because there is a 3.25k Ω resistor between V
OUT
and
calculated slew rate to minimize distortion on the
V
REF
. For a 5V system with AV
DD
/2 = 2.5V, the V
REF
sine wave. For this example, the application
pin buffer must source and sink 2.5V/3.25k Ω = 0.7mA
should only use G = 8, 4.8V
PP
, up to a frequency
minimum for a V
OUT
that can swing from ground to
range of 234kHz to 351kHz, depending upon the
+5V.
acceptable distortion. For a given gain and slew
rate requirement, check for adequate small-signal
bandwidth (typical – 3dB frequency) in order to
assure that the frequency of the signal can be
passed without attenuation.
Table 8. Frequency Response versus Gain (C
L
= 100pF, R
L
= 10k Ω )
0.1% 0.01% 0.1% 0.01%
TYPICAL SLEW SLEW SETTLING SETTLING TYPICAL SLEW SLEW SETTLING SETTLING
– 3dB RATE- RATE- TIME: TIME: SCOPE – 3dB RATE- RATE- TIME: TIME:
BINARY FREQUENCY FALL RISE 4V
PP
4V
PP
GAIN FREQUENCY FALL RISE 4V
PP
4V
PP
GAIN (V/V) (MHz) (V/ µ s) (V/ µ s) ( µ s) ( µ s) (V/V) (MHz) (V/ µ s) (V/ µ s) ( µ s) ( µ s)
1 10 8 3 2 2.55 1 10 8 3 2 2.55
2 3.8 9 6.4 2 2.6 2 3.8 9 6.4 2 2.6
4 2 12.8 10.6 2 2.6 5 1.8 12.8 10.6 2 2.6
8 1.8 12.8 10.6 2 2.6 10 1.8 12.8 10.6 2.2 2.6
16 1.6 12.8 12.8 2.3 2.6 20 1.3 12.8 9.1 2.3 2.8
32 1.8 12.8 13.3 2.3 3 50 0.9 9.1 7.1 2.4 3.8
64 0.6 4 3.5 3 6 100 0.38 4 3.5 4.4 7
128 0.35 2.5 2.5 4.8 8 200 0.23 2.3 2 6.9 10
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