7.5.1 Digital Control of the Gain and Power-Down Pins

The LMH6882 was designed to interface with 2.5-V to 5-V CMOS logic circuits. If operation with 5-V logic is required care should be taken to avoid signal transients exceeding the amplifier supply voltage. Long, unterminated digital signal traces should be avoided. Signal voltages on the logic pins that exceed the device power-supply voltage may trigger ESD protection circuits and cause unreliable operation. Some digital input-output pins have different functions depending on the digital control mode. Table 1 shows the mapping of the digital pins. These functions for each pin will be described in the sections Parallel Interface and SPI-Compatible Serial Interface.

While the full gain range is available in parallel mode both channels must be set to the same gain. If independent channel control is desired, SPI mode must be used.

7.5.2 Parallel Interface

Parallel mode offers the fastest gain update capability with the drawback of requiring the most board space dedicated to control lines. To place the LMH6882 into parallel mode the SPI pin (pin 27) is set to the logical zero state. Alternately, the SPI pin can be connected directly to ground. The SPI pin has a weak internal resistor to ground. If left unconnected, the amplifier will operate in parallel mode.

In parallel mode the gain can be changed in 0.25-dB steps with a 7-bit gain control bus. The attenuator control pins are internally biased to logic high state with weak pull-up resistors, with the exception of D0 (pin 14) which is biased low due to the shared SDO function. If the control bus is left unconnected, the amplifier gain will be set to 6 dB. Table 2 shows the gain of the amplifier when controlled in parallel mode.

The LMH6882 has a 7-bit gain control bus. Data from the gain control pins is immediately sent to the gain circuit (that is, gain is changed immediately). To minimize gain change glitches all gain pins should change at the same time. Gain glitches could result from timing skew between the gain set bits. This is especially the case when a small gain change requires a change in state of three or more gain control pins. If necessary the PDA could be put into a disabled state while the gain pins are reconfigured and then brought active when they have settled.

For fixed-gain applications the attenuator control pins should be connected to the desired logic state instead of relying on the weak internal bias. Data from the gain-control pins directly drive the amplifier gain circuits. To minimize gain change glitches all gain pins should be driven with minimal skew. If gain-pin timing is uncertain, undesirable transients can be avoided by using the shutdown pin to disable the amplifier while the gain is changed. Gain glitches are most likely to occur when multiple bits change value for a small gain change, such as the gain change from 10 dB to 12 dB which requires changing all 4 gain control pins.

A shutdown pin (SD == 0, amplifier on, SD == 1, amplifier off) is provided to reduce power consumption by disabling the highest power portions of the amplifier. The digital control circuit is not shut down and will preserve the last active gain setting during the disabled state. See the Typical Characteristics section for disable and enable timing information. The SD pin is functional in parallel mode only and disabled in serial mode.

Figure 41. Parallel Mode Connection

7.5.3 SPI-Compatible Serial Interface

The serial interface allows a great deal of flexibility in gain programming and reduced board complexity. The LMH6882 serial interface is a generic 4-wire synchronous interface compatible with SPI type interfaces that are used on many microcontrollers and DSP controllers. Using only 4 wires, the SPI mode offers access to the 0.25-dB gain steps of the amplifier.

For systems where gain is changed only infrequently, or where only slower gain changes are required, serial mode is the best choice. To place the LMH6882 into serial mode the SPI pin (Pin 27) should be put into the logic high state. Alternatively the SPI pin can be connected directly to the 5-V supply bus. In this configuration the pins function as shown in Table 2. The SPI interface uses the following signals: clock input (CLK); serial data in (SDI); serial data out (SDO); and serial chip select (CS). The chip-select pin is active low meaning the device is selected when the pin is low.

The SD pin is inactive in the serial mode. This pin can be left disconnected for serial mode. The SPI interface has the ability to shutdown the amplifier without using the SD pin.

The CLK pin is the serial clock pin. It is used to register the input data that is presented on the SDI pin on the rising edge and to source the output data on the SDO pin on the falling edge. The user may disable clock and hold it in the low state, as long as the clock pulse-width minimum specification is not violated when the clock is enabled or disabled. The clock pulse-width minimum is equal to one setup plus one hold time, or 6 ns.

The CS pin is the chip-select pin. This pin is active low; the chip is selected in the logic low state. Each assertion starts a new register access - i.e., the SDATA field protocol is required. The user is required to de-assert this signal after the 16th clock. If the CS pin is de-asserted before the 16th clock, no address or data write will occur. The rising edge captures the address just shifted in and, in the case of a write operation, writes the addressed register. There is a minimum pulse-width requirement for the deasserted pulse - which is specified in the Electrical Characteristics section.

The SDI pin is the input pin for the serial data. It must observe setup / hold requirements with respect to the SCLK. Values can be found in the Electrical Characteristics table (refer to electrical table of the DS). Each write cycle is 16-bit long.

The SDO pin is the data output pin. This output is normally at a high-impedance state, and is driven only when CS is asserted. Upon CS assertion, contents of the register addressed during the first byte are shifted out with the second 8 SCLK falling edges. The SDO pin is a current output and requires external bias resistor to develop the correct logic voltage. See Figure 43 for details on sizing the external bias resistor. Resistor values of 180 Ω to 400 Ω are recommended. The SDO pin can source 10 mA in the logic high state. With a bias resistor of 250 Ω the logic 1 voltage would be 2.5 V. In the logic 0 state, the SDO output is off, and no current flows, so the bias resistor will pull the voltage to 0 V.

Each serial interface access cycle is exactly 16 bits long as shown in Figure 42. Each signal's function is described below. the read timing is shown in Figure 44.

The external bias resistor means that in the high impedance state the SDO pin impedance is equal to the external bias resistor value. If bussing multiple SPI devices make sure that the SDO pins of the other devices can drive the bias resistor.

The serial interface has 6 registers with address [0] to address [6]. Table 4 shows the content of each SPI register. Registers 0 and 1 are read only. Registers 2 through 6 are read/write and control the gain and power of the amplifier. Register contents and functions are detailed below.

The Channel Control register controls how registers 3 and 4 work. When the SYNC bit is set to 1 both channel A and channel B are set to the gain indicated in register 3. When the SYNC bit is set to zero, register 3 controls channel A, and register 4 controls channel B. When the Load A bit is zero data written to register 3 does not transfer to channel A. When the Load A bit is set to 1 the gain of channel A is set equal to the value indicated in register 3. The Load B bit works the same for channel B and register 4.

Figure 42. Serial Interface Protocol (SPI Compatible)

Figure 43. Internal Operation of the SDO Pin

Figure 44. Read Timing