ad8251nbsp Analog Devices, Inc., ad8251nbsp Datasheet - Page 16

• Manufacturer Part Number: ad8251nbsp
• Description: 10 Mhz, 20 V/?s, G = 1, 2, 4, 8 I Cmos Programmable Gain Instrumentation Amplifier
• Manufacturer: Analog Devices, Inc.

AD8251
THEORY OF OPERATION
The AD8251 is a monolithic instrumentation amplifier based
on the classic, three op amp topology, as shown in Figure 49.
It is fabricated on the Analog Devices, Inc. proprietary iCMOS
process that provides precision, linear performance, and a robust
digital interface. A parallel interface allows users to digitally
program gains of 1, 2, 4, and 8. Gain control is achieved by
switching resistors in an internal, precision, resistor array (as
shown in Figure 49). Although the AD8251 has a voltage feed-
back topology, gain bandwidth product increases for gains of 1,
2, and 4 because each gain has its own frequency compensation.
This results in maximum bandwidth at higher gains.
All internal amplifiers employ distortion cancellation circuitry
and achieve high linearity and ultralow THD. Laser trimmed
resistors allow for a maximum gain error of less than 0.03% for
G = 1 and minimum CMRR of 98 dB for G = 8. A pinout opti-
mized for high CMRR over frequency enables the AD8251 to
offer a guaranteed minimum CMRR over frequency of 80 dB at
50 kHz (G = 1). The balanced input reduces the parasitics that,
in the past, had adversely affected CMRR performance.
GAIN SELECTION
This section shows users how to configure the AD8251 for
basic operation. Logic low and logic high voltage limits are
listed in the Specifications section. Typically, logic low is 0 V
and logic high is 5 V; both voltages are measured with respect
to DGND. Refer to the specifications table (Table 2) for the
permissible voltage range of DGND. The gain of the AD8251
can be set using two methods.
+IN
–IN
2.2kΩ
2.2kΩ
+V
–V
+V
–V
S
S
S
S
A1
A2
WR
DIGITAL
GAIN
CONTROL
A0
Figure 49. Simplified Schematic
+V
–V
+V
–V
S
S
S
S
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2.2kΩ
2.2kΩ
+V
–V
+V
–V
10kΩ
10kΩ
S
S
S
S
Transparent Gain Mode
The easiest way to set the gain is to program it directly via a
logic high or logic low voltage applied to A0 and A1. Figure 50
shows an example of this gain setting method, referred to through-
out the data sheet as transparent gain mode. Tie WR to the
negative supply to engage transparent gain mode. In this mode,
any change in voltage applied to A0 and A1 from logic low to
logic high, or vice versa, immediately results in a gain change.
Table 5 is the truth table for transparent gain mode, and Figure 50
shows the AD8251 configured in transparent gain mode.
A1
DGND
NOTE:
1. IN TRANSPARENT GAIN MODE, WR IS TIED TO −V
Figure 50. Transparent Gain Mode, A0 and A1 = High, G = 8
A3
THE VOLTAGE LEVELS ON A0 AND A1 DETERMINE
THE GAIN. IN THIS EXAMPLE, BOTH A0 AND A1 ARE
SET TO LOGIC HIGH, RESULTING IN A GAIN OF 8.
10kΩ
10kΩ
–IN
+IN
10μF
10μF
0.1µF
0.1µF
+V
–V
+V
–V
S
S
S
S
+15V
–15V
OUT
REF
AD8251
DGND
WR
A1
A0
REF
DGND
–15V
+5V
+5V
G = 8
S
.