ISL28617 Datasheet, PDF (14/18 Pages) Intersil Corporation – 40V Precision Instrumentation Amplifier with Differential ADC Driver

ISL28617
The feedback pins V_FB+ and V_FB- have the same input common
mode voltage constraint as the input pins IN+, IN-. The maximum
input voltage range of the feedback pins is given by Equation 11:
V_EE+ 3V < V_CMIRFB < V_CC - 3V
where V_CMIRFB = (+V_OUT - -V_OUT) +V_CMO
(EQ. 11)
To maintain stability, it is critical to respect the ±3V requirement
in Equation 11.
Powering the Rail-to-rail Output Stage
(V_CO, V_EO)
The output stage (A6) is of rail-to-rail design, and is powered by
the V_CO and V_EO pins. The differential output pins +V_OUT, -V_OUT
connect to the V_FB+, V_FB- pins to close the output feedback loop.
The feedback stage is powered from V_CC and V_EE pins. The V_FB+,
V_FB- have a common mode input range 3V below the V_CC rail and
3V above the and V_EE rail. If the output voltage exceeds the
feedback common mode input voltage, loop instability will result.
Therefore, the voltages at the ±V_OUT pins should always be 3V
away from either rail, as shown in Equation 12.
V_EE+ 3V < V_OUT < V_CC - 3V;
where V_OUT = |+V_OUT| or |-V_OUT|
(EQ. 12)
Rail-to-rail Differential ADC Driver
The differential output stage of the ISL28617 is designed to drive
the differential input stage of an ADC. In this configuration, the
V_CO, V_EO power supply pins connect directly to the ADC power
supply pins. This output swing arrangement is ideal for driving
rail-to-rail ADC drive without the possibility of over driving the
ADC input.
The output stage is capable of rail-to-rail operation when V_CO, V_EO
are powered from a single supply or from split supplies. It has a
single supply voltage range (V_CO) from 3V to 15V (with V_EO at
GND), and a ±1.5V to ±15V split supply voltage range. Under all
power supply conditions, V_CC must be greater than V_CO by 3V, and
V_EE must be less than V_EO by 3V to maintain the rail-to-rail output
drive capability.
The V_CMO pin is an input to a very low bias current terminal, and
sets the output common mode reference voltage when driving a
differential input ADC, such that the output would have a ± input
signal span centered around an external DC reference voltage
applied to the V_CMO pin.
Power Supply Voltages by Application
The ISL28617 can be adapted to a wide variety of
instrumentation amplifier applications where the signal source is
powered from supply voltages that are different from the supply
voltages powering downstream circuits. The following examples
are included as a guide to the proper connection and voltages
applied to the supply pins V_CC, V_EE, V_CO and V_EO.
There are a common set of requirements across all power
applications:
1. A common ground connection from the input supplies, (V_CC,
V_EE) to the output supplies (V_CO, V_EO) is required for all
powering options.
2. The signal input pins IN+, IN- cannot float, and must have a DC
return path to ground.
3. The input and output supplies cannot both be operated in
single supply mode due to the 3V feedback amplifier
common mode headroom requirement in Equation 11.
The following are typical power examples:
EXAMPLE 1: BIPOLAR INPUT TO SINGLE SUPPLY
OUTPUT
The ISL28617 is configured as a 5V ADC driver in a high gain
sensor bridge amplifier powered from a ±10V excitation source.
In this application, the ISL28617 must extract the low level
bipolar sensor signal and shift the level to the 0V to +5V
differential rail-to-rail signal needed by the ADC. The following
powering option is recommended:
• V_CC = +10V, V_EE = -10V
• V_CO = +5V, V_EO = GND
• V_CMO = +2.5V
• V_CC, V_EE power supply common connects to GND
EXAMPLE 2: HIGH VOLTAGE BIPOLAR I/O BUFFER
The ISL28617 is configured as a high impedance buffer
instrumentation amplifier in a ±15V industrial sensor
application. In this application, the ISL28617 must extract and
amplify the high impedance sensor signal and send it
downstream to a differential ADC operating from ±15V supplies.
The following powering options are recommended:
1. Input and output supplies are strapped to the same supplies
and rail-to-rail input to the ADC is not required.
- V_CC = V_CO = +15V
- V_EE = V_EO = -15V
- V_CMO = GND
- V_CC, V_EE power supply common connects to GND
and V_OUT = ±12V.
2. ±15V Rail-to-rail output is required, then:
- V_CC = +18V, V_EE= -18V
- V_CO = +15V, V_EO= -15V
- V_CMO = GND
- V_CC, V_EE power supply common connects to GND
The V_CO and V_EO power supply pins connect to the ADC ±15V
power supply pins. Rail-to-rail output swing requires that
V_CC = V_CO +3V and V_EE =V_EO -3V, or ±18V.
14
FN6562.0
May 25, 2012