ISL70617SEHFSLASHPROTO datasheet(21/30 Pages) RENESAS

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ISL70617SEH

FN8697 Rev 5.02

Page 21 of 29

Jul 8, 2022

feedback common-mode input voltage, loop instability will result.

Therefore, the voltages at the ±VOUT pins should always be 3V

away from either rail, as shown in Equation 12.

where VOUT = |+VOUT| or |-VOUT|

Rail-to-Rail Differential ADC Driver

The differential output stage of the ISL70617SEH is designed to

drive the differential input stage of an ADC. In this configuration,

the VCO, VEO power supply pins connect directly to the ADC power

supply pins. This output swing arrangement is ideal for driving

the rail-to-rail ADC drive without the possibility of overdriving the

ADC input.

The output stage is capable of rail-to-rail operation when VCO, VEO

are powered from a single supply or from split supplies. It has a

single supply voltage range (VCO) from 3V to 15V (with VEO at

GND), and a ±1.5V to ±15V split supply voltage range. Under all

power supply conditions, VCC must be greater than VCO by 3V, and

VEE must be less than VEO by 3V to maintain the rail-to-rail output

drive capability.

The VCMO 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 VCMO pin.

Power Supply Voltages by Application

The ISL70617SEH 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 VCC, VEE, VCO, and VEO.

There are a common set of requirements across all power

applications:

1. A common ground connection from the input supplies, (VCC,

VEE) to the output supplies (VCO, VEO) 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 ISL70617SEH is configured as a 5V ADC driver in a high-gain

sensor bridge amplifier powered from a ±10V excitation source.

In this application, the ISL70617SEH 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:

•VCC = +10V, VEE = -10V

•VCO = +5V, VEO = GND

•VCMO = +2.5V

•VCC, VEE power supply common connects to GND

EXAMPLE 2: HIGH VOLTAGE BIPOLAR I/O BUFFER

The ISL70617SEH is configured as a high impedance buffer

instrumentation amplifier in a ±15V industrial sensor

application. In this application, the ISL70617SEH 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.

-VCC = VCO = +15V

-VEE = VEO = -15V

-VCMO = GND

-VCC, VEE power supply common connects to GND

and VOUT = ±12V

2. ±15V rail-to-rail output is required, then:

-VCC = +18V, VEE = -18V

-VCO = +15V, VEO = -15V

-VCMO = GND

-VCC, VEE power supply common connects to GND

The VCO and VEO power supply pins connect to the ADC ±15V

power supply pins. Rail-to-rail output swing requires that

VCC =VCO +3V and VEE = VEO -3V, or ±18V.

EXAMPLE 3: GAINS LESS THAN 1

The ISL70617SEH is configured to a gain of 0.2V/V driving a

rail-to-rail 3V ADC. In this application, the maximum input

dynamic range is ±15V.

•VCC = +18V, VEE = -18V

•VCO = +3V, VEO = GND

•VCMO = +1.5V

•VCC, VEE power supply common connects to GND

In this attenuator configuration, the input signal range is ±15V,

which requires an additional ±3V of input overhead from the

input supplies. Thus, VCC and VEE = ±18V.

AC Performance Considerations

The ISL70617SEH closed loop frequency response is formed by

the feedback GM amplifier and gain resistor RFB and has the

characteristics of a current feedback amplifier. Therefore, the

-3dB gain does not significantly decrease at high gains as is the

case with the constant gain-bandwidth response of the classic

voltage feedback amplifier.

(EQ. 12)

VEE+ 3V < VOUT < VCC - 3V;

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