Circuit Note

CN-0577

analog.com

Rev. 0 | 2 of 10

CIRCUIT DESCRIPTION

ANALOG FRONT END

The most important feature of the CN0577's analog front end (AFE)

is to provide an easy to drive input that can be driven arbitrarily

(single-ended, differential, or pseudo differential), while meeting the

strict drive requirements of the ADC.

The LTC2387-18 SAR ADC requires a fully differential analog input

with a common-mode voltage equal to half of the REFBUF voltage,

which also sets the input range. The CN0577 sets the REFBUF

voltage to 4.096 V, resulting in an input range of ±4.096 V, and a

required common-mode voltage of 2.048 V.

The ADA4945-1 is a low noise, low distortion, fully differential

amplifier (FDA) that is ideal for driving the LTC2387-18. Offset

voltage and drift are 10 µV and 100 nV/°C, respectively; and the

input voltage noise is 1.8 nV/√Hz (full power mode, 100 kHz).

The LTC2387-18 includes a VCM output intended to set the input

common-mode voltage either through a passive network or by

driving a fully differential amplifier's VOCM pin. The VCM output

impedance is 15 Ω and can directly drive the 125 kΩ impedance of

the ADA4595-1's VOCM pin.

The ADA4945-1 input and output pins are DC-coupled, and the

inputs self bias to zero differential voltage when driven through

an external AC coupling capacitor. The ADA4595-1 uses supply

voltages of +6.8 V and -1 V, maximizing the input range and

ensuring that the outputs can utilize the full-scale input range of the

ADC.

As shown in Figure 2, the VOCM pin of the ADA4945-1 is connected

to the VCM pin of the LTC2387-18, and the output is fully differential.

Figure 2. ADA4945-1 Driving the LTC2398-18

Input Range and Common-Mode Range

The ADA4945-1 is configured in unity gain, and since the

LTC2387-18 input range is fixed at ±4.096 V, this is also the input

voltage range of the analog front end. The CN0577 can accept

differential, pseudo differential, or arbitrary input signals.

Figure 3 through Figure 6 show CN0577's input signal capabilities

in different configurations.

Figure 3. Arbitrary Input

Figure 4. Fully Differential Input

Figure 5. Pseudo Differential Bipolar IN- or COM Biased

Figure 6. Pseudo Differential Unipolar IN- or COM Grounded

The limitations on the input common-mode are determined by the

allowable common-mode range at the ADA4945-1 summing nodes

(IN+, IN-), which is specified as -VS to +VS-1.3 V. Arbitrary drive is

not easily analyzed, so the end application's worst-case situations

should be verified using the LTspice® simulation software.