THS1007

SLAS286B − AUGUST 2000− REVISED DECEMBER 2010

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ANALOG INPUT CONFIGURATION AND REFERENCE VOLTAGE

The THS1007 features four analog input channels. These can be configured for either single-ended or

differential operation. Figure 36 shows a simplified model, where a single-ended configuration for channel AINP

is selected. The reference voltages for the ADC itself are VREFP and VREFM (either internal or external reference

voltage). The analog input voltage range is between VREFM to VREFP. This means that VREFM defines the

minimum voltage, and VREFP defines the maximum voltage, which can be applied to the ADC. The internal

reference source provides the voltage VREFM of 1.5 V and the voltage VREFP of 3.5 V (see also section

reference voltage). The resulting analog input voltage swing of 2 V can be expressed by:

VREFM v AINP v VREFP

(1)

10-Bit

ADC

VREFP

VREFM

AINP

Figure 36. Single-Ended Input Stage

A differential operation is desired in many applications due to a better signal-to-noise ration. Figure 37 shows

a simplified model for the analog inputs AINM and AINP, which are configured for differential operation. The

differential operation mode provides in terms of performance benefits over the single-ended mode and is

therefore recommended for best performance. The THS1007 offers 2 differential analog inputs and in the

single-ended mode4 analog inputs. If the analog input architecture is different, common-mode voltages can be

rejected. Additional details for both modes are given below.

10-Bit

ADC

VREFP

VREFM

AINP

Σ

VADC

AINM

+

−

Figure 37. Differential Input Stage

In comparison to the single-ended configuration it can be seen that the voltage VADC, which is applied at the

input of the ADC, is the difference between the input AINP and AINM. The voltage VADC can be calculated as

follows:

VADC + ABS(AINP–AINM)

(2)

An advantage to single-ended operation is that the common-mode voltage

(3)

VCM +

AINM ) AINP

2

can be rejected in the differential configuration, if the following condition for the analog input voltages is true:

(4)

AGND v AINM, AINP v AVDD

(5)

1V v VCM v 4V

SINGLE-ENDED MODE OF OPERATION

The THS1007 can be configured for single-ended operation using dc or ac coupling. In either case, the input

of the THS1007 must be driven from an operational amplifier that does not degrade the ADC performance.

Because the THS1007 operates from a 5-V single supply, it is necessary to level-shift ground-based bipolar

signals to comply with its input requirements. This can be achieved with dc and ac-coupling.