AD7750

–4–

REV. 0

PIN FUNCTION DESCRIPTIONS

Pin

No.

Mnemonic

Descriptions

Power Supply Pin, 5 V nominal

± 5% for specifications.

2

G1

Gain Select, Digital Input. This input selects the gain for the Channel 1 differential input. When G1 is

low, the gain is 1 and when G1 is high, the gain is 16. See Analog Inputs section.

3, 4

Channel 1 Differential Inputs. See the Analog Inputs section for an explanation of the maximum input

signal ranges. Channel 1 has selectable gains of 1 and 16. The absolute maximum rating is

±6 V for each

pin. The recommended clamp voltage for external protection circuitry is

±5 V.

5

AGND

The Analog Ground reference level for Channels 1 and 2 differential input voltages. Absolute voltage

range relative to DGND pin is –20 mV to +20 mV. The Analog Ground of the PCB should be connected

to digital ground by connecting the AGND pin and DGND pin together at the DGND pin.

6, 7

Channel 2 Differential Inputs. See the Analog Inputs section for an explanation of the maximum input

signal ranges. Channel 2 has a fixed gain of 2. The absolute maximum rating is

±6 V for each pin. The

recommended clamp voltage for external protection circuitry is

±5 V.

8

REFOUT

Internal Reference Output. The AD7750 can use either its own internal 2.5 V reference or an external

reference. For operation with the internal reference this pin should be connected to the REFIN pin.

9

REFIN

Reference Input. The AD7750 can use either its own internal 2.5 V reference or an external reference.

For operation with an external reference, a 2.5 V

± 8%, reference should be applied at this pin. For op-

eration with an internal reference, the REFOUT pin should be connected to this input. For both internal

or external reference connections, an input filtering capacitor should be connected between the REFIN

pin and Analog Ground.

10

DGND

The Ground and Substrate Supply Pin, 0 V. This is the reference ground for the digital inputs and out-

puts. These pins should have their own ground return on the PCB, which is joined to the Analog Ground

reference at one point, i.e., the DGND pin.

11

FS

Frequency Select, Digital Input. This input, along with S1 and S2, selects the operating mode of the

AD7750—see Table I.

13, 12

S1, S2

Mode Selection, Digital Inputs. These pins, along with FS, select the operating mode of the AD7750—

see Table I.

14

ACDC

High-Pass Filter Control Digital Input. When this pin is high, the high-pass filter is switched into the

signal path of Channel 1. When this pin is low, the high-pass filter is removed. Note when the filter is off

there is a fixed time delay between channels; this is explained in the Functional Description section.

15

CLKIN

An external clock can be provided at this pin. Alternatively, a crystal can be connected across CLKIN

and CLKOUT for the clock source. The clock frequency is 3.58 MHz for specified operation.

16

CLKOUT

When using a crystal, it must be connected across CLKIN and CLKOUT. The CLKOUT can drive

only one CMOS load when CLKIN is driven externally.

17

REVP

Reverse Polarity, Digital Output. This output becomes active high when the polarity of the signal on

Channel 1 is reversed. This output is reset to zero at power-up. This output becomes active only when

there is a pulse output on F1 or F2. See Reverse Polarity Indicator section.

18

High-Speed Frequency Output. This is also a fixed-width pulse stream that is synchronized to the

AD7750 CLKIN. The frequency is proportional to the product of Channel 1 and Channel 2 or the signal

on either channel, depending on the operating mode—see Table I. The output format is an active high

pulse approximately 90 ms wide—see Digital-to-Frequency Conversion section.

20, 19

F1, F2

Frequency Outputs. F1 and F2 provide fixed-width pulse streams that are synchronized to the AD7750

CLKIN. The frequency is proportional to the product of Channel 1 and Channel 2—see Table I. The

output format is an active low pulse approximately 275 ms wide—see Digital-to-Frequency Conver-

sion section.