WM8170 datasheet(17/41 Pages) WOLFSON

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WM8170

Product Preview Rev 1.0

WOLFSON MICROELECTRONICS LTD

PP Rev 1.0 March 2000

PROGRAMMABLE GAIN AMPLIFIER

The WM8170 contains a Programmable Gain Amplifier (PGA), which precedes the analogue-to-

digital converter (ADC). The gain of the PGA is set digitally via the management interface to a level

which delivers the maximum signal to the input of the ADC without it over ranging, and thus obtaining

the maximum dynamic range from the ADC.

The gain control on the WM8170 is separated into two sections, a programmable gain section, and a

fixed gain section. The programmable gain section is controlled via an 8-bit word written by the

management interface, and has a typical range of between 0dB and 28dB. The gain response of the

programmable gain section is linear on a logarithmic scale. This means that each LSB increase (or

decrease) of digital gain setting represents an equal fraction of a dB (typically 0.11dB) of actual gain

increase (or decrease).

There is also a fixed gain section, which is programmable to be either 0dB or 6dB. Setting the

TIMES2 control bit via the management interface enables this additional gain.

Figure 13 shows the typical WM8170 gain response with and without the additional 6dB.

127

191

255

TIMES2

= 0

PGA CODE

PGA GAIN

(dB)

TIMES2

= 1

Figure 13 Graph of typical WM8170 Gain Response

REFERENCE VOLTAGES

All references used on the WM8170 are derived from an internal bandgap reference voltage. The

ADC uses two reference voltages, VRT and VRB. The Sample/Hold and PGA use a midrail voltage

reference, VMID. The voltage for Reset Level Clamp, VCLP, can be selected to be equal to VRT,

VRB or VMID. These four voltages are buffered on-chip and are each available at output pins. Each

of these pins should be carefully decoupled with capacitors of the type and size detailed in the

Recommended External Components section.

The voltage difference between VRT and VRB can be programmed, in order to accommodate

different input signal ranges, to two values via the management interface. The WM8170 default

condition is VRT-VRB typically 0.5V but can be increased to 0.75V by setting the V375 control bit.

Due to the nature of the ADC design, the difference between VRT and VRB is typically half the

maximum input signal which the ADC can convert successfully, i.e. if VRT-VRB is 0.75V, then the

ADC can accommodate an input signal after the PGA of greater than 1.5V.

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