Datasheet

MAX9939
SPI Programmable-Gain Amplifier
with Input V
OS
Trim and Output Op Amp
9
Maxim Integrated
SHDN: Set SHDN to 0 for normal operation. Set SHDN
to 1 to place the device in a low-power 13µA shutdown
mode. In shutdown mode, the outputs OUTA and OUTB
are high impedance, however, the SPI decode circuitry
is still active. Each instruction requires a write to the
SHDN bit.
MEAS: The MAX9939 provides a means for measuring
its own input offset voltage. When MEAS is set to 1, the
INA- input is disconnected from the input signal path
and internally shorted to INA+. This architecture thus
allows the input common-mode voltage to be compen-
sated at the application-specific input common-mode
voltage of interest. The input offset voltage of the PGA
is the output offset voltage divided by the programmed
gain without any V
OS
trim (i.e., V3:V0 set to 0):
V
OS-INHERENT
= (V
OUTA
- V
CC
/2)/Gain
Program V
OS
to offset V
OS-INHERENT
. The input V
OS
also includes the effect of mismatches in the resistor-
dividers. Setting MEAS to 0 switches the inputs back to
the signals on INA+ and INA-. Each instruction requires
a write to the MEAS bit.
Programming Gain
The PGA’s gain is set by the bits G3:G0 in the Gain reg-
ister. Table 3 shows the relationship between the bits
G3:G0 and the amplifier’s gain. The slew rate and
small-signal bandwidth (SSBW) of the PGA depend on
its gain setting as shown in Table 3.
Programming Input Offset Voltage (V
OS
)
The input offset voltage is set by the bits V4:V0 in the
Input Offset Voltage Trim register. Bit V4 determines the
polarity of the offset. Setting V4 to 0 makes the offset
positive, while setting V4 to 1 makes the offset negative.
Table 4 shows the relationship between V3:V0 and V
OS
.
To determine the effect of V
OS
at the output of the ampli-
fier for gains other than 1, use the following formula:
V
OUTA
= V
CC
/2 + Gain x (V
OS-INHERENT
+ V
OS
)
where V
OS-INHERENT
is the inherent input offset voltage
of the amplifier, which can be measured by setting
MEAS to 1.
Applications Information
Use of Output Amplifier as Active Filter
The output amplifier can be configured as a multiple-
feedback active filter as shown in Figure 3, which tradi-
tionally has better stopband attenuation characteristics
than Sallen-Key filters. These filters also possess inher-
ently better distortion performance since there are no
common-mode induced effects (i.e., the common-
mode voltage of the operational amplifier is always
fixed at V
CC
/2 instead of it being signal dependent
such as in Sallen-Key filters). Choose external resistors
and capacitors to create lowpass, bandpass, or high-
pass filters.
G3:G0
GAIN
(V/V)
SLEW RATE
(V/µs)
SMALL-SIGNAL BANDWIDTH (MHz)
0000 1 2.90 2.15
0001 10 8.99 2.40
0010 20 8.70 1.95
0011 30 12.80 3.40
0100 40 12.50 2.15
0101 60 13.31 2.60
0110 80 12.15 1.91
0111 120 18.53 2.30
1000 157 16.49 1.78
0.2 (V
CC
= 5V)
1001
0.25 (V
CC
= 3.3V)
2.86 1.95
1010 1 2.90 2.15
Table 3. Gain