Texas Instruments ADS5102 EVM, ADS5103 EVM manual Circuit Function, Analog Inputs

Circuit Function

3.1 Circuit Function

The following paragraphs describe the function of the individual circuits. See the data sheet for device operating characteristics.

3.1.1Analog Inputs

The ADC has either transformer-coupled inputs or differential-amplifier inputs from a single-ended source. The inputs are provided via the SMA connectors

J1 and J2 on the EVM, which must be configured as follows:

-For a differential amplifier input to the ADC, a single ended source is

connected to J1. R36 and R38 must be installed, and R37 and R39 must be removed. The input has a 50-Ω terminator.

-For a 1:1 transformer coupled input to the ADC, a single ended source is

connected to J2. R36 and R38 must be removed, and R37 and R39 must be installed. The input is ac-coupled and has a 50-Ω terminator.

3.1.2External Reference Inputs

In addition to being able to use the internal reference of the ADC, a reference circuit has been included on the EVM. This circuit uses a precision 2.5-V, low-noise linear regulator as the primary source, and allows adjustment of the REFT and REFB signals to the ADC using potentiometers R27 and R28, respectively. A third source, CML, is also generated to provide an adjustable common mode voltage to be used by the transformer and differential amplifier during external reference operation. CML is adjusted by potentiometer R29. In order to use the ADC with external references, install jumpers W10 and W11, install jumper W3 between pins 2 and 3, jumper W6 between pins 1 and 2, and jumper W1 between pins 1 and 2. If REFT is set to any voltage other than 1.25 V, jumper W1 must be installed between pins 2 and 3 for optimal ADC performance. The ranges of the external reference signals are shown in Table 3–1.

Table 3–1. Reference Voltage Adjustment Ranges

Signal	Minimum Voltage	Typical Voltage	Maximum Voltage
REFT	0.9	1.25	1.6
REFB	0.3	0.75	0.9
CML	0.5	1.0	1.25

3-2